Solved: thanks, Kevin. Your observation prompted me to actually look at the folder, which I should have done before posting. I had forgotten that I had moved the folder years ago into Google Drive (now drive G:) with a link to it from where it normally is. This was in aid of sharing database, media and settings between computers via cloud sync. Database on OneDrive because it would not sync as long as RM had the file open and the rest on Google Drive. The link was broken because GDrive wasn’t running. Once started, no more error.
A puzzling and disturbing error message when I first tried to open the link said that Google Backup and Replace was no longer working or available (something like that) and did I want to uninstall it! That was disconcerting - would uninstalling it lose the data? I didn’t and was relieved to see G: reappear. I probably should revert to normal as I haven’t been switching computers for some time now.