I tried your suggestions. But now I'm in a world of claws-hurt!

I tried rebuilding the folder tree. It ran for quite some time (2 hours) and then bailed out. The result was that while subfolders were now visible, only a small portion of the folder tree was visible and (most importantly!) my Inbox was no longer visible.

After playing around with it with no success, I decided to just uninstalll and re-install Claws-mail and start over again. But when I reinstall Claws, it comes back with the previous configuration rather than a clean slate! I even tried deleting the folders that Claws-mail created, uninstall it, reboot the machine, and re-install. It still doesn't come back clean.

So, now I have no idea what to do.