After disabling ipv6 I was able to install apps and adjust the domain of the dashboard.
This has fixed the problem for me. Since i have no need for using ipv6.
I use even more cloudron servers. I checked whether it was using IPv6. Now I saw that IPv6 was also turned off on all these servers. I've never turned it on myself. By any chance has it recently been turned on by default? The server with the problems is the first server I installed since cloudron version 8.
But thank you very much for your time @joseph @nebulon @luckow