Cache has already been cleared. Tried on both firefox and opera on ubuntu and on firefox on a iphone. Mobile does not seem to suffer from this issue.
Firefox seems to keep "hanging". Opera starts with the same issue, but recovers after ~7-8 sec and seems to do a hard refresh (which solves the problem).
My feeling is saying that a local IP address from a DNS resolve is kind of "wrong", maybe a browser does not like that. I tried experimenting with hairpinning and that kind of stuff too.
I installed ubuntu directly on a system and cloudron on top. 16GB, 4 cores and 256gb storage. VM was the first thought, but installed directly to compare the behavior.
I also installed cloudron on a EC2 instance. No problemo....