Rocket Chat /rocketchat not available sometimes



Rocket Chat is installed -> no changes made.

Normally when I surf to:
I successfully get redirected to /rocketchat/home where I get my login-screen.

But sometimes this doesn’t work and I get:

Not Found

The requested URL /rocketchat was not found on this server.

Apache/2.4.25 (Univention) Server at subdomain.mydomain.tld Port 443

how to forcefully get this error:
create a google-chrome -app of “subdomain.mydomain.tld/rocketchat”