Cannot call sendError() after the response has been committed on Synology

[Jens_Gwerder]

I'm currently trying to set up confluence with a postgresql datbase on my Synology DS918+ NAS.
Mostly I'm following this tutorial "https://www.giuliomenna.net/confluence-on-synology-nas-via-docker-and-with-lets-encrypt-ssl/".

The problem now is, that I'm getting an HTTP Status 500 – Internal Server Error Cannot call sendError() after the response has been committed error.

I'm running confluence behind a reverse proxy.

The other threads about this error didn't help me out and the confluence knowledge help page didn't really help me out...

You will find the whole error log in the attachment, oh yeah, atlassian doesn't have an attachment system...

I've uploaded it to this website:
https://send.firefox.com/download/e2bf971a72e1abd4/#cV4pCbbuAF1_Dlsb4dzJuw 

What could be the problem?

Thank you