Internal server error after login

Hi all!

I know I am not the first one with this problem but I couldn’t find a solution to solve it so far…

I have Nextcloud running on a raspberry pi and it worked just fine until today. When I enter the IP-Adress I saw the login-page, but with the “normal” background, although that I changed it before. When I tried to log in, I got the error message:

Internal Server Error

The server encountered an internal error and was unable to complete your request.
Please contact the server administrator if this error reappears multiple times, please include the technical details below in your report.
More details can be found in the server log.

Although that I didn’t change anything, I cannot log in now. I am not used to work with unix so I need detailed instructions on how I can resolve this problem.

The data of nextcloud is stored on an external harddrive and the installed version of Nextcloud is

This is the error log of apache2 (I stopped and restarted it):
[Wed Jan 10 17:55:36.760203 2018] [mpm_prefork:notice] [pid 657] AH00163: Apache/2.4.25 (Raspbia$
[Wed Jan 10 17:55:36.761307 2018] [core:notice] [pid 657] AH00094: Command line: '/usr/sbin/apac$
[Wed Jan 10 20:31:53.629640 2018] [mpm_prefork:notice] [pid 657] AH00169: caught SIGTERM, shutti$
[Wed Jan 10 20:32:07.091842 2018] [mpm_prefork:notice] [pid 8593] AH00163: Apache/2.4.25 (Raspbi$
[Wed Jan 10 20:32:07.092058 2018] [core:notice] [pid 8593] AH00094: Command line: '/usr/sbin/apa$

I really need your help! Thanks a lot!