Hey captiva.
Check that the timing belt tensioner is tensioned clockwise before tightening the nut. It sounds like its tensioned the wrong way (ie) anti clockwise. I have seen this done before, it allows the belt to be in contact with the thermostat housing enough to cause damage.
Clockwise Allows the belt to be lifted away from the thermostat housing by the tensioner.
Let me know if this helps.

Regards, Stephen.