OK, I was having some major problems after upgrading back in November 2006. Backups worn't working at all, just a blank page, kept getting login errors, you name it. So being that it was hard getting help I did a fresh brand new install of 1.7.2 ONLY and one by one installed the extra's that I wanted in my school. EVERYTHING works perfectly!!
Only problem is I'm having a hard time transferring the users over as well as their assignments, I transferred the database information over to the new database, etc. . . to no good.
But at least I have it working. Moodle 1.7.2, CertificateV2, Activity Locking, Internalmail. Going to work at adding other features.
My suggestion to anyone trying it like this. . . backup your database and directory EACH step of the way. If you had a backup problem, try it after each addon is installed if all goes well, back everything up again. If you have any problems you can revert to before the errors happened.
Good luck.