I just joined the club

Some USSD codes did not work on my N900 so I installed ussd4all and tried - didn't work either. Then the phone app kept crashing. I tried to reboot - and reboot, reboot, reboot...

It turned out that ussd4all tries to copy and patch
/usr/lib/librtcom-call-ui.so.0.0.0 as
/usr/lib/librtcom-call-ui.so.0.0.0-p1
and rewrites the symlink librtcom-call-ui.so.0 to the patched version of the library. If patching fails the patched variant is never created and the symlink is broken afterwards.

Maybe these steps help some poor soul who tried the same:

First we have to enable R&D mode and disable the "lifeguard reset" which reboots the device because the phone system process dies:
1 connect N900 via USB
2. slide out the keyboard
3. turn off the phone (power button long press - if you can manage it) and turn it on again pressing "u" on the keyboard
4. "NOKIA" white screen appears and shows USB logo in upper right corner
5. on the PC invoke
flasher-3.5 --enable-rd-mode --set-rd-flags=no-lifeguard-reset

Disconnect USB and boot your N900. That should work as usual now. The phone app still crashes.

Maybe you should remove ussd4all now. You can do this but it won't fix the phone ui.

Fix the broken library symlink:
open xterm
cd /usr/lib
ls -l librtcom-call-ui*
# see the link to lib....-p1 ? ...-p1 does not exist? fine!
ln -sf librtcom-call-ui.so.0.0.0 librtcom-call-ui.so.0

Reboot and check that your phone ui works again - it should!

Finally disable R&D again:
same procedure as above this time with
flasher-3.5 --disable-rd-mode --clear-rd-flags=no-lifeguard-reset

unplug, reboot - be happy! :-)

PS: I am *not* responsible for any damages - if you brick your device (more than it already is...) then you are on your own!