Multiple installations of RM on your hard drive -or- any/last installation going to a USB/removable device can result in a mismatch of the last stored information necessary to upgrade.
Only one installation, never installed to a USB/removable drive.
I did check the RootsMagic.ini file and the paths were all empty. To be extra safe, I started RootsMagic and went to Tools, Options, Folders and selected the path for the RootsMagic item, saved then exited RootsMagic and tried the update again - no good, still got the error.
I could uninstall and reinstall, which I'll do eventually if I can't find the problem and no one has any more suggestions, but working with computers for a living I like to know WHY a piece of software isn't working as it should.