Hi Matthias....
MY OPINION..... I would suspect that if you found a problem with your clamps, that the pressure in the radiator system was not the culprit - that they would have come off no matter what the cap pressure rating is.
As I look at it - the standard rating cap at 13 PSI is NOT supposed to be venting all the time.
If you have a good cooling system (which we know you do), then the pressure in the system will typically be less than 13 psi.
If that weren't the case (and the 13 PSI caps were constantly venting) - then Pantera overflow tanks would constantly be filling up, and owners would have to keep refilling their systems. That is generally not the case in a well constructed cooling system like yours.
I thought you said your system was running at a reasonable temp (less than 200*F or 93*C)….
NEVERTHELESS - USA radiator caps are cheap (for us) and I am certainly not suggesting that you keep running a 20 PSI cap.
MY OPINION - I don't think the cap was the problem - it was hoses/clamps.
Which hose came off? Most of the hoses are can't pop off (but they can leak)because there are metal tubes on both sides. The outlet of the water pump (or is it the inlet?) is tricky because the size changes.
Viel Gluck!
Chuck