OK. I have some more diagnostics.
- nuveau was already uninstalled
- created an xorg log with 4 monitors (crashes X) and an xorg log with just 2 monitors (works)
startx in runlevel 3 with 4 monitors gives this log (crash):
https://paste.opensuse.org/13486293
startx in runlevel 3 with 2 monitors gives this log (works):
https://paste.opensuse.org/15650184
Same hardware, same configuration (4 monitors) works fine with OpenSuse 42.3
Any ideas anyone?