reginos:
The cap has been redesigned years ago and from what I know there are no problems.
Perhaps in the case of our friend the specific part on his car had a defect and needed to be replaced.
I will check with the Service Rep. as he wasn't there when the car was ready and I never actually saw the work order. He had given me the information about the cap verbally.
I also posted this same issue on the 6speed forum and received one other interesting response which referred to the problem being caused by a bad water pump seal. The owner in this case had the seal replaced and it fixed the problem.
I don't know how thoroughly Porsche service technicians check problems like this, i.e. tests, etc. but one would think they would at minimum have checked all the parts connected to the coolant system.