I too have an LPKF 91s and I'm trying to get it to work. I read through your post and unfortunately I don't have a solution for your buffer overrun problem. I do however have the pinout for the DB9 to DB25 connector (should be attached to this post). I got this from LPKF's usa branch. Hope it helps.
I also had a question that I was hoping you could answer. I'm unable to get my machine to talk to my computer over the com port. I start up boardmaster (im using 5.0 by the way) and it tells me to turn on the machine, which i do, but then it just keeps telling me to turn on the machine. I've scoped the com lines coming out of the computer and it looks as though the computer is trying to talk with the machine, but the machine is not responding. When I turn on the machine, it zero's itself in the x and y direction and then moves to the tool change position. From this I know that at least the machines motor driver boards are working. I've checked the com port settings on the computer (8N1, 9600 baud rate, etc..) and they all match what LPKF says to use.