“MainWP Child plugin not detected or could not be reached! Ensure the MainWP Child plugin is installed and activated on the child site, and there are no security rules blocking requests. If you continue experiencing this issue, check the”
Problem solved. It was the Premium Starter Templates-plugin from Beaver Builder that caused the problem. I deactivated it, updated it and after that it was working. I’m not using that plugin so i ended up deleting it.
Whats wrong with that plugin is out of my knowledge.