Thank you for the replies! At last I have checked the physical connections once again and it must have been a short somewhere as rebinding the wires eliminated these issues (at least part of them).
It turned out that the permanent errors that the Leaf had caused by a dead fuse and this is caused probably because of an insulation issue among the wires.
Although CORYFLOWER CAN library still causes temporary errorcodes in the car but after switching to another one (SANDEEP MISTRY) the errors don't appear anymore.
Now my problem is that although I receive CAN messages after starting the device but sniffing the messages either stops after an initial 20-30printed messages or only prints RTR message after this initially printed 20-30messages. I am not even sure if this initial messages are correct as if I print out the data in char format it is visualized as garbage (random ASCII characters not even numbers).
I have doublechecked the connections and the wiring but these seems to be ok. Do you have a gint on what can be wrong here?
Thank you!