@chriszang and another update.
So I am happily working on my CAN receiver sketch and just noticed that I forgot to press the button that grounds INT-output of the mc2515 but the upload still worked. Gave it another try (w/o pressing the button) and it's still working - weird.
Then I swapped this COMMU module for my 2nd COMMU Module and here the upload doesn't work.
My explanation is that the grounding of the mcp2515 INT-output basically created a short circuit in the transistor that controls this pin and eventually burned it out. Not a big problem for me as
a) I am not using interrupts in my sketch
b) I have a 2nd COMMU unit
I just wanted to warn you guys that grounding the INT output has unintended consequences.