Even if it’s SSL certificate issue ot should affect the working, right?
You completely ignored the support template so we really have no idea since we need more information to help you.
I’m not not clear how you’re “syncing” your photos. There are multiple ways of settings things up so please give us the step-by-step of your process.
Are you using a web browser? Or are you using one of the official Nextcloud mobile client apps? (If so, which one? Android? iOS? And which version?)
Where are you seeing the “Internal Server error 500” appear part way through your transfers? Maybe you can post a screenshot?
Are the “server logs” you posted in your follow-up from the actual container/VM/whatever running PHP/Nextcloud? They look more like they’re from a reverse proxy terminating just your HTTPS perhaps?