If you still haven't solved the problem: my Grohe thermostatic shower tap has at each inlet (hot and cold) a small filter. These are designed to block any debris entering the tap from demaging the ceramic disks. These filters -small meshs- can get blocked with debris, limescale etc.. So you dismantel the shower tap and take them out, clean them under running water with a nailbrush and put them back in.
The other problem could be that somehow the fixed setting of the thermostat is out of control, read the manual to find out how to reset it. If you haven't got the manual anymore ask at Grohe for a new one.