Broken GS308EPP web gui after reverse proxy

Hello community,

I put my switch GS308EPP behind a reverse proxy HTTP -> HTTPS on my Synology DS923+ which works well.

Unfortunately, the errors come after the login.

The first error which I solved was that I could not log me in, because the credential mask infinitely loops. This I could manage when setting a proxy header "Cookie $http_cookie" in my reverse proxy settings on my Synology.

The second error which I struggle with is, that when logging me in, the whole web UI looks broken.

Starting the F12 developer mode in Google Chrome, it says that some .js functions and .svg vectors get 502 - Bad Gateway.
Here I got stuck.

To make it even a little bit easier, I published some screenshots showing my problem in detail.