Thanks to all who responded.

So typical of this community... I try to give up, but nooooo, now I have to try again :-) , and you've convinced me, I'll try again tonight.

In short, I tried everything (and read the threads, so I think I'm up to date). U-Boot without CSSU Thumb works, but if I have CSSU Thumb installed it doesn't, and I need to flash the kernel as otherwise it goes into a reboot loop. Granted, I've never tried installing U-Boot first, as the first thing I do after reflash is to set up CSSU. I also played around quite a bit with Backupmenu, so that might be a culprit as well (I did always remove it prior to U-boot install, but still...).

So consider this thread closed, I'll probably be asking for help in the U-Boot and CSSU Thumb threads respectively, that is the right place to do so. And again, thanks for the morale boost.