license keep disappearing at random time

Hej I have Mathcad 8.0.0.0

the problem is that every time I type in my license it works, but after a random time it disappears. then I have to type it in again. I have tried getting a new license(didn't work), I have also tried reinstalling, mathcad, but I still have the problem.

does anyone have a clue about what I can do?

it says this every time the license disappears 

lincese status: invalid

license type: no license