Now I have figured out why MCP2515 stopped outputting the messages (why it stopped working): it does not like to be fed from a voltage regulator... When I powered it from the arduino I have tested with eventually then it worked like a charm. I do not really understand why this is the case? Arduino provides 4.95V, voltage regulator 5.05V. MCP should be good up to 5.5V - according to the datasheet...