Office on Mac asks to activate very often

 I have an active Office 365 subscription. I've installed Office on my Mac 3 weeks ago, but its keeps asking me to activate. When I try signing in, it says password or user ID in incorrect. I can sign in to Office 365 on my PC and to the Office 365 portal using the same credentials. So there is nothing wrong with the user ID or password. 

This is very frustrating. Please advice.