Copilot Solo was also in vogue in the Argosy force in 1966, when I started, but I've no recollection of any such sortie from around mid-67, when I guess it must have been phased out across the MRT force.
But back to WW2 and the Halifax. The Handley Page history by C H Barnes covers Halifax fin development in detail, and mentions "rudder stalling which caused spiral instability with two engines dead on one side and had led to many crashes on final approach at night." Pretty much as Franek mentioned, above. Several shapes were investigated and that finally used on later Mk IIs and production Mk IIIs had a fin area some 50% larger than on early aircraft.