OK, I tried several different dm9601.c found over the net but none worked. At last I went back to vanilla kernel and took the dm9601 from version 2.6.24 and with this one the blue adapter seems to work well (it fixed the MTU overrun at two places).

Unfortunately I have got something messed up in the meantime - now an ICD2 process eats 100% CPU time and JFFS2 reports something about missing disk space (df shows enough free disk). I don't know if/how it is related to the new kernel module so let me fix that before announcing total success with the DM9601...