It was with the Microsoft Store app. I now uninstalled it, so not sure which version it was, but it was the latest one, whatever was available a few days ago.

Seems that my image link was not preserved in the post. The error text was:
"Cannot save home in <path to the file>" java.lang ...

I want to base my design on an existing sample project. So I do File -> New from demo -> 4-Bedroom house

Then I try to save it but keep getting an error:
https://imgur.com/XvHXdB9

I'm able to save a completely empty project though.

Also, the same error happens if I copy everything from the 4 ...