For EU project we were using Nextcloud to store and share data.
It worked two weeks ago. Today we all receive: HTTP Error 500: Diese Seite funktioniert nicht deep-purple.eu kann diese Anfrage momentan nicht verarbeiten.
Because of the url of your Nextcloud it seems that you use your webspace (subdir and database)) for your Nextcloud. That is not the best possibility for a Nextcloud installation. Your host seems to be HostEurope. Ask the person who installed the Nextcloud fore details. Mostly it is better to use a Managed Nextcloud. But it seems that HostEurope does not offer it.