I had this too, after installing my Mediator 4000. It worked fine several years but with the Mediator it randomly gave this error until it warmed up and then after a reset it suddenly worked. I could cure it by putting it into another Zorro slot.
Also there are some Zorro cards known to conflict with the Highway. Try to use the Highway alone and put it into the first Zorro slot. If it works, add the other cards one at a time to check if there are any conflicts.
KDH might not have time to answer mails because they are moving. According to their homepage they will be back to normal after September 13th.
However, the USB mailing list mentioned by Doobrey is the better place to ask, because the manufacturer of the Highway and the author of Poseidon read and answer there.
Bye,
Thomas
