I have closed upstream bug: https://gitlab.freedesktop.org/xorg/lib/libx11/-/issues/186 since i got no feedback at all and it seems affecting only the specific libX11 1.8.4 - fvwm2 combination that very few people use, I think.
The dirty fix for me was to switch from fvwm2 to the developing fvwm3 window manager. I don't know if the window manager crash is due to a libX11 bug or to library API misuse by fvwm2. The problem appeared after a recent libX11 update (1.8.3 -> 1.8.4) while fvwm2 is frozen since long time. Stefan
