So a little update.
I reflashed my device in good hopes with pr 1.3, but still got the error.
Then after reading this thread, just applied a little pressure to the upper right area under the battery where supposedly the gsm modem is situated. After reboot, no more error.
So this is a hardware issue for me atleast. my phones HW type is RX-51.
Tired of carrying my old C6-00 which is a poor substitute for my n900, so I guess, I have to carry around two. 1 for fiddling around and one for calling.

Edit
Tried to reproduce the issue, and I can say that, it happens from regular use, while holding the device in two hands and opening the keyboard, then applying only a little pressure from both sides, can easily make this HW defect reappear and after some more handling disappear again. So a cold solder joint or worn out contacts somewhere. Pity, that I do not have the equipment to attempt to resolder the modem part.