I have Office:Mac 2008 nicely running on a G4 tower with 10.4.11 from 2004, and Entourage works fine. 2008 was the last of the Office version the support PowerPC Macs. We also have been able to use the much older older Office X on an older eMac and, again, Entourage works fine. So I don't think it's a version issue with Entourage.
Can you access either account through your browser? That's important to check that the accounts are active and your ISP is doing its job.
Allow me to suggest a forum more targetsed to Entourage--Microsoft's Office :Mac forums. They are very helpful and MS Mac Unit employees often help out there: