Openproject - Error 413

I have a fresh UCS Installation with only Openproject and all Updates installed.
When trying to upload an Attachment i get HTTP Error 413 Request entity too large.

The Attachment is only 1 MB in size

Error Message within Openproject:
Ein interner Fehler ist aufgetreten. Http failure response for https://ucs.*******.com/openproject/api/v3/attachments: 413 OK

Can anyone point me to a solution?

Nevermind, it was a wrong configuration in my NGINX Reverse Proxy