Here's the Apache log:
[Wed Aug 02 21:26:22.222057 2017] [:error] [pid 14720] [client 109.78.8.138:49617] PHP Warning: rename(/var/www/forum.vaerlingus.com/vendor2,/var/www/forum.vaerlingus.com/vendor): Directory not empty in /var/www/forum.vaerlingus.com/vendor/illuminate/filesystem/Filesystem.php on line 146, referer: https://forum.vaerlingus.com/admin
[Wed Aug 02 21:26:22.257872 2017] [:error] [pid 14720] [client 109.78.8.138:49617] PHP Fatal error: Uncaught Error: Class 'GuzzleHttp\\RequestOptions' not found in /var/www/forum.vaerlingus.com/vendor/guzzlehttp/guzzle/src/Client.php:130\nStack trace:\n#0 /var/www/forum.vaerlingus.com/vendor/guzzlehttp/guzzle/src/Client.php(89): GuzzleHttp\\Client->request('get', 'packages/pogode...', Array)\n#1 /var/www/forum.vaerlingus.com/vendor/flagrow/bazaar/src/Repositories/ExtensionRepository.php(117): GuzzleHttp\\Client->__call('get', Array)\n#2 /var/www/forum.vaerlingus.com/vendor/flagrow/bazaar/src/Repositories/ExtensionRepository.php(166): Flagrow\\Bazaar\\Repositories\\ExtensionRepository->getExtension('pogodevorg$flar...')\n#3 /var/www/forum.vaerlingus.com/vendor/flagrow/bazaar/src/Api/Controllers/InstallExtensionController.php(49): Flagrow\\Bazaar\\Repositories\\ExtensionRepository->installExtension('pogodevorg/flar...')\n#4 /var/www/forum.vaerlingus.com/vendor/flarum/core/src/Api/Controller/AbstractSerializeController.php(97): Flagrow\\Bazaar\\Api\\Controllers\\InstallExtensionController->data(Object(Zend\\Stratigility\\Http\\Requ in /var/www/forum.vaerlingus.com/vendor/guzzlehttp/guzzle/src/Client.php on line 130, referer: https://forum.vaerlingus.com/admin
[Wed Aug 02 21:26:22.279577 2017] [:error] [pid 14721] [client 109.78.8.138:49619] PHP Fatal error: Uncaught Error: Class 'Flarum\\Http\\AccessToken' not found in /var/www/forum.vaerlingus.com/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php:29\nStack trace:\n#0 /var/www/forum.vaerlingus.com/vendor/zendframework/zend-stratigility/src/Dispatch.php(212): Flarum\\Http\\Middleware\\RememberFromCookie->__invoke(Object(Zend\\Stratigility\\Http\\Request), Object(Zend\\Stratigility\\Http\\Response), Object(Zend\\Stratigility\\Next))\n#1 /var/www/forum.vaerlingus.com/vendor/zendframework/zend-stratigility/src/Dispatch.php(88): Zend\\Stratigility\\Dispatch->dispatchCallableMiddleware(Object(Flarum\\Http\\Middleware\\RememberFromCookie), Object(Zend\\Stratigility\\Next), Object(Zend\\Stratigility\\Http\\Request), Object(Zend\\Stratigility\\Http\\Response), NULL)\n#2 /var/www/forum.vaerlingus.com/vendor/zendframework/zend-stratigility/src/Next.php(160): Zend\\Stratigility\\Dispatch->__invoke(Object(Zend\\Stratigility\\Route), NULL, Object(Zend\\Stratigility\\Http\\Request), Object(Zend\\Stratigility\\Http\\Response), Object(Zend\\Stratigility\\Nex in /var/www/forum.vaerlingus.com/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php on line 29, referer: https://forum.vaerlingus.com/admin
[Wed Aug 02 21:26:22.281918 2017] [:error] [pid 14891] [client 109.78.8.138:49621] PHP Fatal error: Uncaught Error: Class 'Flarum\\Http\\AccessToken' not found in /var/www/forum.vaerlingus.com/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php:29\nStack trace:\n#0 /var/www/forum.vaerlingus.com/vendor/zendframework/zend-stratigility/src/Dispatch.php(212): Flarum\\Http\\Middleware\\RememberFromCookie->__invoke(Object(Zend\\Stratigility\\Http\\Request), Object(Zend\\Stratigility\\Http\\Response), Object(Zend\\Stratigility\\Next))\n#1 /var/www/forum.vaerlingus.com/vendor/zendframework/zend-stratigility/src/Dispatch.php(88): Zend\\Stratigility\\Dispatch->dispatchCallableMiddleware(Object(Flarum\\Http\\Middleware\\RememberFromCookie), Object(Zend\\Stratigility\\Next), Object(Zend\\Stratigility\\Http\\Request), Object(Zend\\Stratigility\\Http\\Response), NULL)\n#2 /var/www/forum.vaerlingus.com/vendor/zendframework/zend-stratigility/src/Next.php(160): Zend\\Stratigility\\Dispatch->__invoke(Object(Zend\\Stratigility\\Route), NULL, Object(Zend\\Stratigility\\Http\\Request), Object(Zend\\Stratigility\\Http\\Response), Object(Zend\\Stratigility\\Nex in /var/www/forum.vaerlingus.com/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php on line 29
[Wed Aug 02 21:26:22.283952 2017] [:error] [pid 16084] [client 109.78.8.138:49622] PHP Fatal error: Uncaught Error: Class 'Flarum\\Http\\AccessToken' not found in /var/www/forum.vaerlingus.com/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php:29\nStack trace:\n#0 /var/www/forum.vaerlingus.com/vendor/zendframework/zend-stratigility/src/Dispatch.php(212): Flarum\\Http\\Middleware\\RememberFromCookie->__invoke(Object(Zend\\Stratigility\\Http\\Request), Object(Zend\\Stratigility\\Http\\Response), Object(Zend\\Stratigility\\Next))\n#1 /var/www/forum.vaerlingus.com/vendor/zendframework/zend-stratigility/src/Dispatch.php(88): Zend\\Stratigility\\Dispatch->dispatchCallableMiddleware(Object(Flarum\\Http\\Middleware\\RememberFromCookie), Object(Zend\\Stratigility\\Next), Object(Zend\\Stratigility\\Http\\Request), Object(Zend\\Stratigility\\Http\\Response), NULL)\n#2 /var/www/forum.vaerlingus.com/vendor/zendframework/zend-stratigility/src/Next.php(160): Zend\\Stratigility\\Dispatch->__invoke(Object(Zend\\Stratigility\\Route), NULL, Object(Zend\\Stratigility\\Http\\Request), Object(Zend\\Stratigility\\Http\\Response), Object(Zend\\Stratigility\\Nex in /var/www/forum.vaerlingus.com/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php on line 29
[Wed Aug 02 21:26:22.288146 2017] [:error] [pid 14647] [client 109.78.8.138:49626] PHP Fatal error: Uncaught Error: Class 'Flarum\\Http\\AccessToken' not found in /var/www/forum.vaerlingus.com/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php:29\nStack trace:\n#0 /var/www/forum.vaerlingus.com/vendor/zendframework/zend-stratigility/src/Dispatch.php(212): Flarum\\Http\\Middleware\\RememberFromCookie->__invoke(Object(Zend\\Stratigility\\Http\\Request), Object(Zend\\Stratigility\\Http\\Response), Object(Zend\\Stratigility\\Next))\n#1 /var/www/forum.vaerlingus.com/vendor/zendframework/zend-stratigility/src/Dispatch.php(88): Zend\\Stratigility\\Dispatch->dispatchCallableMiddleware(Object(Flarum\\Http\\Middleware\\RememberFromCookie), Object(Zend\\Stratigility\\Next), Object(Zend\\Stratigility\\Http\\Request), Object(Zend\\Stratigility\\Http\\Response), NULL)\n#2 /var/www/forum.vaerlingus.com/vendor/zendframework/zend-stratigility/src/Next.php(160): Zend\\Stratigility\\Dispatch->__invoke(Object(Zend\\Stratigility\\Route), NULL, Object(Zend\\Stratigility\\Http\\Request), Object(Zend\\Stratigility\\Http\\Response), Object(Zend\\Stratigility\\Nex in /var/www/forum.vaerlingus.com/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php on line 29
[Wed Aug 02 21:26:22.291917 2017] [:error] [pid 14820] [client 109.78.8.138:49623] PHP Fatal error: Uncaught Error: Class 'Flarum\\Http\\AccessToken' not found in /var/www/forum.vaerlingus.com/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php:29\nStack trace:\n#0 /var/www/forum.vaerlingus.com/vendor/zendframework/zend-stratigility/src/Dispatch.php(212): Flarum\\Http\\Middleware\\RememberFromCookie->__invoke(Object(Zend\\Stratigility\\Http\\Request), Object(Zend\\Stratigility\\Http\\Response), Object(Zend\\Stratigility\\Next))\n#1 /var/www/forum.vaerlingus.com/vendor/zendframework/zend-stratigility/src/Dispatch.php(88): Zend\\Stratigility\\Dispatch->dispatchCallableMiddleware(Object(Flarum\\Http\\Middleware\\RememberFromCookie), Object(Zend\\Stratigility\\Next), Object(Zend\\Stratigility\\Http\\Request), Object(Zend\\Stratigility\\Http\\Response), NULL)\n#2 /var/www/forum.vaerlingus.com/vendor/zendframework/zend-stratigility/src/Next.php(160): Zend\\Stratigility\\Dispatch->__invoke(Object(Zend\\Stratigility\\Route), NULL, Object(Zend\\Stratigility\\Http\\Request), Object(Zend\\Stratigility\\Http\\Response), Object(Zend\\Stratigility\\Nex in /var/www/forum.vaerlingus.com/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php on line 29
[Wed Aug 02 21:28:01.538377 2017] [:error] [pid 15026] [client 109.78.8.138:49639] PHP Warning: require(vendor/autoload.php): failed to open stream: No such file or directory in /var/www/forum.vaerlingus.com/admin.php on line 12
[Wed Aug 02 21:28:01.540819 2017] [:error] [pid 15026] [client 109.78.8.138:49639] PHP Fatal error: require(): Failed opening required 'vendor/autoload.php' (include_path='.:/usr/share/php') in /var/www/forum.vaerlingus.com/admin.php on line 12
[Wed Aug 02 21:28:04.010775 2017] [:error] [pid 14646] [client 109.78.8.138:49640] PHP Warning: require(vendor/autoload.php): failed to open stream: No such file or directory in /var/www/forum.vaerlingus.com/index.php on line 12
[Wed Aug 02 21:28:04.010858 2017] [:error] [pid 14646] [client 109.78.8.138:49640] PHP Fatal error: require(): Failed opening required 'vendor/autoload.php' (include_path='.:/usr/share/php') in /var/www/forum.vaerlingus.com/index.php on line 12
[Wed Aug 02 21:28:21.444921 2017] [:error] [pid 14720] [client 109.78.8.138:49645] PHP Warning: require(vendor/autoload.php): failed to open stream: No such file or directory in /var/www/forum.vaerlingus.com/index.php on line 12
[Wed Aug 02 21:28:21.445162 2017] [:error] [pid 14720] [client 109.78.8.138:49645] PHP Fatal error: require(): Failed opening required 'vendor/autoload.php' (include_path='.:/usr/share/php') in /var/www/forum.vaerlingus.com/index.php on line 12