FINAL FIX!
First - THANK YOU to everyone who made comments and helped in the process!
Whenever I research something, very rarely people post the final conclusion or the reason for what they had trouble with. I won‘t do that!
Problem - Engine overheating!
Cause - radiator drained almost empty due to a leak at the heater control valve (underneath the cab)
Solution - refill coolant!
Problem - NO heat in cab!
Cause - bad heater control valve
Solution - replaced valve!
Short summary of events:
After using my skidsteer for about 25-30hrs this summer after having a head gasket replaced from the local CAT dealer, I parked it in perfect working condition. And I did not use cab heat at all.
When trying to run it with heat this winter, both of above problems occurred.
Refilling solved the overheating, but a slow drain of coolant and the lack of any heat sustained.
By doing a pressure test of the coolant system, I pretty quickly found the puddle of coolant underneath the cab in the very front.
The actual part what leaked, was the valve body at the part where the black plastic connects to the heater core brass/copper part (not sure what material it is).
Comments:
The heater control valve is TERRIBLE in the way how it connects and how flimsy it is. We were unable to actually install the two bolts which hold the control valve body firm in place. After one hour, we gave up on it due to being very cold and just let it sit in there unsecured. Fingers crossed!