Problem uploading .bin files on Raspberry Pi

Hello, I am running osmocom-bb on a Ubuntu x86 machine with a Motorola C121. Everything works (as far as I can tell). Now I would like to move the host programs to a Raspberry Pi. So, I did a "make nofirmware" on the Rpi and copied the *.compalram.bin files from the x86 Ubuntu box over to the Rpi. When I use osmocon on the pi to upload the bin file to the phone, the transfer stops after the "finished" line and then, after a while, the phone sends a DOWNLOAD NAK, instead of the usual "DOWNLOAD ACK your code is running now". Has anybody seen this behaviour and knows how to fix it? I have tried two USB/serial adapters (both FTDI based) just in case its a hw problem, but the result is always the same: the upload to the phone works on the Ubuntu/x86 box but not on the pi. Any help is greatly appreciated. Kind Regards, -Walter