I discussed this problem in a previous post
KM Bug? Export All Macros as Folder – Failed to Export - Questions & Suggestions - Keyboard Maestro Discourse
and the solution was to save to a non dropbox directory.
The problem recurred and I spent the day trying to find the issue.
Restarting the mac, KM engine and editor and changing directory did not work.
I ran @DanThomas 's excellent backup macro which also did not work BUT listed the name of the one of 5,000+ problematic macro. It appears that the title of the macro contains a troublesome character.
I simply changed the title of that macro and now everything works.
Without @DanThomas 's macro, I would never have found the issue and would have had to forfeit Export All Macros as Folder permanently.
I checked the engine log and the problematic macro is not identified.
Solution: Export All Macros as Folder should tell the user which macro is problematic instead of simply failing.