I contacted the Fujifilm support in Germany, the problem is apparently known. The flash shoe on the GFX50S (on the camera body, when the viewfinder is removed) is not identical to the flash shoe on the viewfinder, so no trigger, splitter or similar should be used here, as there is a risk that it cannot be removed, as in my case. Now I have sent the camera in and I hope to get it back soon so that I can work with it again. Unfortunately, this is not mentioned anywhere in the manual. But maybe one or the other will benefit from my experience.