Hello Danut,
I was able to set up a Low Speed CAN card to talk to a High speed port on another computer( simulating a high speed device). I did not have any termination resistors on the cable. Initially, I saw the same Comm errors you mentioned and this was because the cable I was using was internally terminated. So please check that the cables are not terminated. Also , verify that the high speed device itself is not terminated ( measure the resistance between CAN_H and CAN_L, with the cable connected and without the cable).
Let me know if you were able to get the system up and running. Also do you have any low speed devices that can be used to test teh CAN card with?