I’m using Firefox. I cleared the cache and cookies (Preferences -> Cookies and Site Data -> Clear Data), quit Firefox, logged out, logged in again, opened Firefox and tried to access Kiwitrees -> same error messages.
I’m very embarrassed, it seems like I’m the only person who regularly has problems with upgrades.
By the way, I’m using a fairly old version of Php (version 5.6.30). I don’t know whether this could be significant.
Thanks a lot for your attempts to sort out the problem.