Yahoo also caused similar problems to Gmail. How do you use Yahoo? IM? No problems there. Yahoo!
No problem there. Did you set up Yahoo as Email client using BB Wizard?
That is where I am talking about Also, if you click on Options/Advanced Options/Default Services, normally BIS shows up for both Calendar and Messaging. But after installing Yahoo and Gmail as Email clients in Wizard Setup/Email settings, I get Yahoo, Gmail and BIS as my default services which creates many problems such as loosing all of my contacts, etc.