Microsoft Communicator for Mac

The company I'm currently working for uses Dell laptops with Microsoft Office Communicator as their IM. I have a Macbook Pro and have installed Communicator for Mac. It persists in either logging out constantly or refusing to log in, despite the password being correct.

It's very unstable - making my work day very hard work. Has anyone else come across this problem and do you know how to fix it?

I'm using version 13.1.3.

Many thanks,

Phil