First attempt resulted in error: new update available.
I misunderstood this (was meant with respect to the update plugin, if I remember correctly) and wondered why I could not get the direct link to this newer version. So I just went ahead, files were downloaded and at some later step the process hung.
Having experienced this several times, I just repeated the process. Eventually I noticed I had to update the plugin first. So I did.
Now the result is:
Hotfix release available: 2018-04-22b "Greebo". upgrade now! [50.2] (what's this?)
start
Wiki Upgrade
Checking file permissions...
/doku.php will be updated.
What to do here? PC has been cold started, Windows 10, 8 GB RAM, Firefox 66.0.4.