as I said Jacob, such isteacher() error seems to be part of a global change done some months ago. Perhaps you stopped upgrading your Moodle server just in the middle of it?
I think that the correct solution should be to upgrade to 1.4.5+ (a lot of thing have been improved since your 1.4.3+ version) and then try it again.
If your problem remains with the new version you can send me privately a copy of your backup file to examine it in my test servers...
Ciao