We also had this issue when we upgrade to Officescan 11 SP1, we are working with Trend for couple of months with no resolutions. Workaround, we unload Officescan agent on the computer, wait for 5 minutes and try opening any of the Office apps and it shouldn't ask for activation anymore, then you can enable Officescan Agent. Sometime rebooting also help.
We also experienced, opening different Office App also help, for example, you get message of unlicensed in Excel, close Excel and open Word or some other app and then it doesn't ask for activation. Then you can reopen Excel without any issues.
We provide image of our computer to Trend, they ran multiple utilities still they are not able to provide with any fix. We end up donwgraded most of desktop to Officescan 10.6.