Check out my Poseidon tutorial on my website http://www.hd-zone.com under the AmigaOS 4.1 Classic section.
It may help.
Thank you HammerD, I tried something that I had forgotten : installing poseidon on 4.1.3 (4.1 was a clear install, wiped out 4.0)
The thing is that I remember poseidon surviving resets and 3.9 to 4.0 boots (it was flashed to the ALGOR's EPROM).
Without changing a thing, after the update I can move the USB mouse pointer while on the 4.1.3 early startup-menu , but shortly after choosing the boot device (the 4.1.3 WB) it won`t boot to WB (black screen). If I disable the whole flashrom content, it boots fine.
I loaded the previous modules as were before, leaving out everything Poseidon-related stuff and I got it to boot! I think there must be a conflict with the WB (I`ve already commented out the USB kickstart modules and the "USBCtrl start" command).