Having different problems to access a Mattermost server including 500 errors

Question

We have some problems with our Mattermost server depending the access and uploading files and or saving settings. The person that set up the whole system isn't accessible, so I tried to fix it.
The intial problem was that we couldn't send .jpg and .pdf files (Other files like e.g. .png files are sometimes working) and getting some server erros. Plus our version wasn't supported anymore so I upgraded the server on a newer version but that didn't fixed the problem, we just got one or two more problems.

Complete working system: Mattermost server 5.37.0 running on AWS with a nginx proxy on another AWS server.
Changes made: Generated new SSL certificate on the main domain but nothing on the subdomains, updated the server to 6.3.9.

Problem 1: Different devices can't load the main.js file when they want to login. Instead they see a white screen. The error code in the console is "Failed to load resource: ERR_EMPTY_RESPONSE" for the main.js and main.css file. After a few hours till one day the client is loading the files and you can access the server. From the mobile clients you can access the server all the time. When you delete the cache and reload the desktop app you getting again a white screen. Looks like the desktop client has to wait to load something into the cache?

Problem 2: I can't save any settings in the backend of the client. Every time I try to save something I get the following error: PUT https://chat.domain.tdl/api/v4/config 500 (Internal Server Error) main.d54d4ec050258613a991.js:2
I already looked up the permission on the config.file, it's -rw-rw-r-- Looks good for me.

Problem 3: Is similiar to the second problem. We can't send any .jpg and .pdf files to each other (Some .png or .svg files are working). The error code in this case: https://chat.domain.tdl/api/v4/files 500 (Internal Server Error). This problem was as stated earlier before the update on the new version of the server.

I already spent some time reading different sites, questions & answers but didn't found anything that could help me. I'm thankful for every hint I can get in this case.