no expansion boards involved, just regular UTP cat 5e cable extension via regular RJ-45 connectors.
I ended up replacing the two RJ-45 heads and it now works again. I think it is a humidity problem. Wish I put a whole length of cable instead of those two lengths and RJ-45 joints. Oh well… just ordered some waterproof RJ-45 female-female connector from ebay. Hope it will do the trick. Otherwise I shall be whining here for a shovel-free method of replacing those two lengths of cable with one piece. Wish me luck!