I did submit a bug report, and had the following kind statement from the developers

"Thanks for reporting. This problem is caused by a bug in the update process of our installers.

I'm afraid the only solution is for you to uninstall Spyder and manually install 6.0.5. That way Spyder will work again without any issues. You can download that version from:

https://github.com/spyder-ide/spyder/releases/tag/v6.0.5

We're really sorry for the inconvenience."

I followed this advice, and this worked well.