This may help others....
In an attempt to isolate the export problem, I created about 60 tags and attached each one to no more than 100 notes. I then selected notes using the new tags, one by one, and exported. Of the roughly 60 exports, one failed with 'There was a problem...' BUT when i repeated the export, twice, it worked both times.
So I conclude the original failure was not an un-exportable note, but some other problem. I suppose the 'Please try again' suggestion in the Evernote error message might work - and for modest note volumes be practical. (My two online attempts to export the full notebook of 6000 notes ran for over an hour before failing)
Later, for the hell of it, I re-tried the original 6000 note export and it succeeded, so the error was transient.