"At home it was plugged into the car connector. "
Yes, but only getting it's power from that.
The Null-Modem cable was plugged directly into the empeg.

This had worked before (I had an empeg sled in my desktop for almost 18months), but I had that ser_rat=115000 there and always upgraded via ftp.

So yes, the FAQ entry could have save me a lot of trouble....