Server showed as online on the hosting provider dashboard but it wasn't responding. After contacting support, this was fixed at some point.

This led to a config file being reverted because of a setting I didn't change earlier which led to the page only showing "Server error."

Everything should work again and in the future that config file shouldn't change by itself anymore. I still have no idea what the problem on the providers side was, let's hope it doesn't happen again :P