Internal server problem

When I want to log into my cloud, it says this:

The server could not complete the request.
If this occurs again, please send the technical details below to your server administrator.
Further details can be found in the server log.

What do I have to do to be able to log in again?

Look in /path/to/nextcloud/data/nextcloud.log and also in your webserver log. Have you installed all needed packages?