In a chat session via https://getsupport.apple.com/ProductSelector.action a first Apple support representative advised me to turn off encryption in recovery mode, but that function was greyed out. So I could not use that solution.
Then I opened a second chat session with another support representative, who was a very nice woman who concluded that this was a problem better solved on the phone. She directed me to set up a telephone appointment. Every time my issue was redirected to another Apple representative, they knew about my case right away, by using a support ticket number. In the confirmation e-mail I received I found the appointment in a iCal file. Very convenient.
On the assigned time, my phone rang. The guy I spoke to soon recognized he could not resolve this particular problem himself, but he had heard about it. It seems it is a problem many people have encountered. He indicated that system engineers are working on it, and that the problem may be resolved in an upcoming system update. He gave me good news and bad news… Good: the problem can be solved, bad: the solution includes erasing the drive. The process involved some low level terminal stuff in recovery mode. Just DIskUtility to erase the main volume is not sufficient. So I got transferred to some high level system engineer who talked me through wiping the drive with some Terminal magic. The Apple engineer also confirmed some people were working on it to solve the problem. Then I restored my machine using one of the Time Machine backups I ran just before. All in all that took a few hours at night.
The problem is now solved, although I now have no FileVault encryption. I will consider turning it back on when it‘s updated.
All in all, unlike some in this thread, my experience with Apple support is quite positive. All the people I spoke to were very professional, polite and knowledgeable. No machine of this complexity is perfect, all in all, my uptime percentage has been only just below 100% in more than 25 years of using Apple products. So this glitch is no biggie for me.