Thank you for your post.
This issue occurs under Mac OS in that all copies and versions of FileMaker Pro use the same preferences file, and this message can occur when more than one copy of FileMaker Pro is installed on the machine. Is this the case?
Weird, I thought I replied but now I don't see the reply.
Yes, this was probably the case. For testing purposes I had 15v1, 15v2 and 15v3 installed on my Mac at the same time.
Not saying it's not a bug but at least not as serious as it seemed at first sight ;-)