No errors in the flarum log. In Apache2's log is the following.
[Mon Jan 14 16:05:54.954633 2019] [php7:warn] [pid 19614] [client 75.134.195.75:56678] PHP Warning: file_put_contents(/var/www/html/public/../storage/sessions/D9dMxFy2KWtotSbnm8nOlEDV2tcVVXWFNK7xLgs9): failed to open stream: Permission denied in /var/www/html/vendor/illuminate/filesystem/Filesystem.php on line 122, referer: http://142.93.242.224/
[Mon Jan 14 16:06:32.617792 2019] [php7:warn] [pid 19611] [client 75.134.195.75:56683] PHP Warning: file_put_contents(/var/www/html/public/../storage/sessions/D9dMxFy2KWtotSbnm8nOlEDV2tcVVXWFNK7xLgs9): failed to open stream: Permission denied in /var/www/html/vendor/illuminate/filesystem/Filesystem.php on line 122, referer: http://142.93.242.224/
[Mon Jan 14 16:06:54.585916 2019] [php7:warn] [pid 19613] [client 75.134.195.75:56689] PHP Warning: file_put_contents(/var/www/html/public/../storage/sessions/D9dMxFy2KWtotSbnm8nOlEDV2tcVVXWFNK7xLgs9): failed to open stream: Permission denied in /var/www/html/vendor/illuminate/filesystem/Filesystem.php on line 122, referer: http://142.93.242.224/
[Mon Jan 14 16:07:33.511810 2019] [php7:warn] [pid 19624] [client 75.134.195.75:56694] PHP Warning: file_put_contents(/var/www/html/public/../storage/sessions/NkwpP0gkKKGyzrOfgDWa3eJdn2IbZ8OtEINtrRZS): failed to open stream: Permission denied in /var/www/html/vendor/illuminate/filesystem/Filesystem.php on line 122, referer: http://142.93.242.224/public/
[Mon Jan 14 16:07:33.870546 2019] [php7:warn] [pid 19624] [client 75.134.195.75:56694] PHP Warning: file_put_contents(/var/www/html/public/../storage/formatter/Renderer_8d9401eca742fa939e0eba0026ecf0e6bfa8d32f.php): failed to open stream: Permission denied in /var/www/html/vendor/s9e/text-formatter/src/Configurator.php on line 5214, referer: http://142.93.242.224/public/
[Mon Jan 14 16:07:33.870602 2019] [php7:warn] [pid 19624] [client 75.134.195.75:56694] PHP Warning: include(/var/www/html/public/../storage/formatter/Renderer_8d9401eca742fa939e0eba0026ecf0e6bfa8d32f.php): failed to open stream: No such file or directory in /var/www/html/vendor/s9e/text-formatter/src/Configurator.php on line 5217, referer: http://142.93.242.224/public/
[Mon Jan 14 16:07:33.870612 2019] [php7:warn] [pid 19624] [client 75.134.195.75:56694] PHP Warning: include(): Failed opening '/var/www/html/public/../storage/formatter/Renderer_8d9401eca742fa939e0eba0026ecf0e6bfa8d32f.php' for inclusion (include_path='.:/usr/share/php') in /var/www/html/vendor/s9e/text-formatter/src/Configurator.php on line 5217, referer: http://142.93.242.224/public/
[Mon Jan 14 16:07:33.871435 2019] [php7:error] [pid 19624] [client 75.134.195.75:56694] PHP Fatal error: Uncaught UnexpectedValueException: The stream or file "/var/www/html/public/../storage/logs/flarum-2019-01-14.log" could not be opened: failed to open stream: Permission denied in /var/www/html/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php:107\nStack trace:\n#0 /var/www/html/vendor/monolog/monolog/src/Monolog/Handler/RotatingFileHandler.php(119): Monolog\\Handler\\StreamHandler->write(Array)\n#1 /var/www/html/vendor/monolog/monolog/src/Monolog/Handler/AbstractProcessingHandler.php(39): Monolog\\Handler\\RotatingFileHandler->write(Array)\n#2 /var/www/html/vendor/monolog/monolog/src/Monolog/Logger.php(344): Monolog\\Handler\\AbstractProcessingHandler->handle(Array)\n#3 /var/www/html/vendor/monolog/monolog/src/Monolog/Logger.php(707): Monolog\\Logger->addRecord(400, Object(Error), Array)\n#4 /var/www/html/vendor/flarum/core/src/Http/Middleware/HandleErrorsWithView.php(87): Monolog\\Logger->error(Object(Error))\n#5 /var/www/html/vendor/flarum/core/src/Http/Middleware/HandleErrorsWithView.php(69): Flarum\\Http\\Midd in /var/www/html/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php on line 107, referer: http://142.93.242.224/public/