If you are running NextCloud version 12 and you cannot reach the “Admin” area of the Web GUI because you receive an error message page that states that an Internal Server Error has occurred, here is a solution to this problem that worked for me.
Solution: Try disabling and “removing” the OnlyOffice App. Merely disabling the OnlyOffice App did not help. Login as a user with administrative privileges, go to Apps, go to “office and text” apps category page, and click the “remove” link under the “OnlyOffice” app. Now, go to the admin area and you will be granted access to the admin console area pages.
Not to worry. The OnlyOffice selection will remain in the apps area. When you click the button to “Enable” OnlyOffice, the enabling process will automatically download / import the necessary files (to connect NextCloud to your physically separate OnlyOffice server) that were previously removed.
I had installed Nextcloud version 11 on Ubuntu 16.04 and PHP 7.x without issue and within a few days received an update to Nexcloud version 12 which update I applied and it has been working fine. I first tried to install OnlyOffice and later tried to install Collabora Online, neither of which worked for me, so I scrapped the idea of using those apps, and I disabled both Apps in the Apps management area of the Nextcloud Web GUI. However, I recently noticed that I cannot access the Admin Console pages in the web GUI. Everything else in the Nextcloud web GUI can be accessed without error messages, but when I select “Admin” I get the following 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.
The same issue and solution were found with the OnlyOffice App in NextCloud Version 11 as well. See: https://help.nextcloud.com/t/nextcloud-11-cant-access-admin-console/11533