After some more attempts today I've been able to catch a bugreport + dump.
Repro steps:
1. booted into my WB (RTG 1080p on nVidia gtx 980m 368.39, Picasso 96 built-in emu, full-window RTG, fullscreen for native)
2. changed WB3.9 Screenmode to Super Hires
3. opened a shell and sent a "C:Reboot" command
the Amiga rebooted and the crash-dump warning appeared right after opening reloaded WB screen by Startup-Sequence...
http://www43.zippyshare.com/v/WrnFib0a/file.html