'Undefined variable' error while trying to solve in Mathcad 14

I really hope someone can help me with this problem. Suddenly my mathcad started to du something weird. When trying to solve an equation involving something I have defined earlier I always get the message "this variable is undefined". But if I then just insert the size I have defined as numbers instead it works fine ... and it happens every time I try something similar. I don't know what to do, and i already tried reinstalling several times..