I have the same issue as @youngtk96 with 500 Internal Server Error.
In the error logs, I find the following.
PHP message: PHP Catchable fatal error: Argument 3 passed to Flagrow\Upload\Events\Adapter\Identified::__construct() must be an instance of Flagrow\Upload\Contracts\UploadAdapter, null given, called in /var/www/html/vendor/flagrow/upload/src/Commands/UploadHandler.php on line 110 and defined in /var/www/html/vendor/flagrow/upload/src/Events/Adapter/Identified.php on line 31" while reading response header from upstream, client: X, server: hidden-address-here, request: "POST /api/flagrow/upload HTTP/1.1", upstream: "fastcgi://unix:/var/run/php5-fpm.sock:", host: "laddtryck.se", referrer: "hidden-address-here"
I have replaced IP and address in the error message above, fyi.