Solved
I seem to have fixed it myself. I reset the RCC configuration using the Reset Configuration button, enabled the RCC global interrupt, and changed to the Clock Configuration. I now got the pop-up dialog asking whether I wanted to allow clock issues to be resolved, which I did. I used the Resolve Clock Issue button and voila!, the RTC clock was enabled. SYSCLK was reduced to 32 MHz, but I might be able to live with that. After a successful test, I turned off the RCC global interrupt and the build continued to work.
Seems like a missed opportunity to inform the developer.
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