You didn't have this problem before you uninstalled the threadsintegration? Is it the same for all users? Did you use the "Merge User Databases" functionality when you installed the system? Can you log in to the Administrator? Are your uses showing up in the User list in the Mambo Administrator? Have you tried clearing all cookies before logging in again?
Uninstalling does absolutely nothing with the Mambo database, so it seems odd that the behavior should change.