Phone sync failure: "Invalid username/password"

Yes, I have entered the correct username and password. I have not been able to perform my first sync however, with my Sony Ericsson M600 ( M600i ). Here's what happens:

After following the initial sync instructions to a T, setting up the Funambol account correctly, WITH an updated and triple-checked username and password, I ask my contacts to sync on my phone. The process begins, goes smoothly as it begins to connect via AT&T's MediaNet service. Finally, the process terminates and says that it failed because of an "Invalid username/password"

Any ideas? Need more information from me? Please help if you can, or post something if you have a similar problem.

Best,
Felipe