I've now reflashed and reverted back to the 4-1 version and all seems to be working.

Same here, 4-1 works whereas 4-2 fails to boot any kernel including the u-boot's attached kernel...

I have 4-2 and attached kernel (PKv51) boots with no problems

Even I am having problems booting with 4-2.I tried everythhing,but seems to remain that way.Have to reflash it then....

Hello, I am also having problems with the U-boot. Installed the two packages from the first post and after that I installed the Linux kernel for power user (boot image for multiboot) package.

After this the bootmenu appears, but although I can see the Internal Nand and Maemo 5 with kernel-power ... options no one will boot. Tho boot starts, but hangs with the Starting kernel ... message. After some time the phones tunrs off.

I also can not access the backupmenu to flash my backup - I made before - on the device (N900). I also tried to start Nemo from the SD card, the boot starts, goes after the Starting kernel message but hangs afterward with kernel panic - not syncing: No init found.

So is there anything I can try ? At least to get to the backupmenu.

Re-flash phone -> install Backupmenu -> recover your backup.

P.S.
I'm always reading this thread, because I'm also interested in having U-Boot + Backupmenu + NITDroid, but I'm reading about these problems with the new version of U-Boot...

@karlitos
re-flash the kernel (flasher-3.5 -F <firmware-image> --flash-only=kernel -f -R) this replaces the u-boot with Nokia stock kernel..

Hmm ... is there any other way how could I at least access the filesystem ? I am not sure if backupmenu stores all files I have on my phone. I was still counting with the possibility to access the filesystem over the cable with backupmenu.

Ou! great, and than ? How can I go back to the powerkernel ? But at least it would allow me to access the filesystem right ?

As far as I know, Backup-menu doens't make a backup of mydocuments folder.
Can't you connect it in mass storage mode?

You can (re)install the power kernel from H.A.M.

As long as you don't do a complete flash (just the kernel) you won't loose a thing except for the non working u-boot, just uninstall u-boot from ham and it should be all good..