Hi Howard,
Good tip. I am helping out a new moodler (and learning that I have not made every mistake possible) on a 1.9.9 site.
My tip is to look at the /auth directory for folders that do not belong there. Sometimes the ones that do not belong there are obvious. Maybe archive the suspected folders, then delete those misplaced folders
For example, another auth subdirectory might cause a problem. Not sure how this happened on this particular site. Yesterday, i found a /mod/mod (which created a debug error because it was looking for the "mod" activity). Saw your tip this morning, and had an AH HA. Yep there was an /auth/auth folder with all the subfolders under it too! Deleting it got rid of the error message and the site.
I think I will let the owner of the site get rid of any other double folders and stop my volunteer work. Well off to work seeing if I can avoid breaking a old and tired 1.5.2 server while repairing some backup MySQL tables.
Bless you Howard! No MySQL dump and rebuild needed. And upgrading to 1.9.10 would not have fixed that problem either those extra folder would remain. Heros all. Chris