Undo history failed.

All good, did you get a chance to make the screenshot? I don't expect to see anything wrong so really just a sanity check. Also, Reaper can make backups of undo files automatically IIRC, that won't help for a very recent undo, but it might be good for troubleshooting. I have had a very weird undo thing occur a couple years ago but only twice, my gut feeling is that in my case something like a script, corner-case combo of custom actions, or SWS corrupted the undo state. But I can't be sure.