The most recent official diplomacy rules state:
"Disrupting a convoy. Dislodgment of a fleet in a convoy causes the convoy to fail. If a Fleet ordered to convoy is dislodged during the turn, the Army to be convoyed remains in it's original province. An attack on a convoying Fleet, which does not dislodge it, does not affect the convoy."
The phpdiplomacy adjudicator is completely getting this rule wrong. One unsupported fleet on holland disrupted my Yorkshire-Norway convoy for North Sea, and I'm without a first turn build! Alas!
Autumn 1901, Diplomacy: Your fleet at North Sea was attacked by a fleet at Holland, and had to defend itself so couldn't convoy.
Autumn 1901, Diplomacy: The fleet at North Sea isn't accepting the convoy request from the army at Yorkshire, so the convoy order failed.
Autumn 1901, Diplomacy: Your fleet at London attempted to move into English Channel but there was a stalemate.
Autumn 1901, Diplomacy: Your fleet at North Sea was attacked by the fleet at Holland, but sucessfully defended.