Yes, I have the exact same problem. Just over an hour ago I was playing The Last Days of the Third Age mod for Warband when it crashed. I went to restart Warband and continue but it updated the game first and now I have this popup saying I have a trial version and need to input an activation key on the taleworlds site.
I've had Mount and blade warband in my steam library for many years and have over 650 hours in it suddenly is now a 'trial version'?