Problem solved. Seems the 2 lights failing on one corner were not related to each other. I have disconnected all plugs - sprayed them with WD40 and re-connected them -> several times. Doing that I have found a new better methode to remove the fog light:

instead of wiggling around from the front through the hole in the bumper you can grab it from the engine bay side and push it through - much easier then the other methode.

Thanks TerminatorX5 - I was afraid to have a problem in the cabling or a plug in between the cabling - but since you told me that cabling goes straight through I was feeling better :-)

Best, Helmuth