Taken apart the OBC,.
Looks like I got a fried capacitor:
hmmm, need to figure out how to post image, google photo link does not seem to work...
It is the top blue one in the doghouse surrounded by the black conformal coating on this picture (this is not mine, copied from upthread):
The old errors are still there too.
The communication problems are probably due to the broken data connector (1 pin is missing):
That pin just fell off when I was lifting the board up.
The old battery was a bit low (60% SOC, was reading 12.8V), but it seems to be still OK, I just charged it up without problem.
So I think it was just low because it did not get charge due to the DC-DC not working.