Unable to accede folder and files after short time (http/https?)

Have following problem on nextcloud 11 and 12:
After some time (hours) with fresh installations I

  1. with 12: login link not clickable, if logged in, menue not clickable
  2. with 11/12: after some time (and multiple browsers) with click on menue point “file” unable to reach folder structure, files:

Link, when it is working http://cloud.plusreal.at/index.php/apps/files/?dir=/&
Link, when it is not working: https://cloud.plusreal.at/index.php/apps/files/#

tried already to find a solution in forum. the problem is if you are inexpert, you don´t even know where and with which search words.

thx for any help

Nextcloud version (eg, 10.0.2): 12.0.0
Operating system and version (eg, Ubuntu 16.04):ubuntu
Apache or nginx version (eg, Apache 2.4.25):
PHP version (eg, 5.6): 5.6.30
Is this the first time you’ve seen this error?:

Can you reliably replicate it? (If so, please outline steps):

The issue you are facing:

The output of your Nextcloud log in Admin > Logging:

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

The output of your Apache/nginx/system log in /var/log/____:

Remember, this information may be requested if it isn’t supplied; for fastest response please provide as much as you can :heart: Feel free to use a pastebin service, otherwise log files can be indented with 4 spaces on each line to present them in a friendlier way on the forum.