What German torpedo problem was discovered in 1942?

As I think this may be the answer to your question what problem was discovered in 1942 related to the torpedos hydrogen venting system. The problem was that when vented anywhere but on the surface (at 1 atmosphere) it disturbed the mechanism used to control the depth the torpedo ran at causing torpedos to run deep.

"...was the discovery of the last major defect in the standard German torpedo and the resulting perfection of the torpedo’s depth-keeping device. Ironically, this discovery did not come from the technical minds at the TI, but was rather the accidental result of an unscheduled on-board inspection of torpedoes performed by one of the youngest skippers in the Kriegsmarine. On January 31, 1942, U-94, a Type VIIC submarine, was on her way back to Germany after being forced to abort her patrol due to mechanical difficulties. On the return journey the boat’s young skipper, twenty-three year old Otto Ites, decided to make an unscheduled thorough inspection of his boat’s torpedoes. While ventilating one of the weapons, Ites noticed that a leaky seal allowed an unusual amount of air pressure to build up inside the torpedo’s balance chamber in which the depth control mechanism was located. Finding this odd, Ites surfaced to report his findings to BdU. Coincidentally, this report was received by Dönitz at the very time that his staff was puzzling over the large number of torpedo failures recently reported by the first boats arriving off the North American coast. Upon receiving Ites’ transmission, Dönitz quickly recognized the potentially negative implications this pressure build could have on the performance of the depth-keeping mechanism and immediately forbade the on-board ventilating and heating of G7e torpedoes" (1)

This was because inside the torpedo was an "atmospheric chamber" that contained pressure at 1 atmosphere, the difference between actual pressure and that control pressure was used by the submarine to determine depth of the torpedos run. When venting the hydrogen while submerged it disturbed this control chamber and altered the pressure slightly but just enough that when the torpedo compared actual pressure to the control it would be off and run deep.

Heinz Trompelt, the torpedo mechanic on U172 described the issue as,

"The depth control container on the torpedo protruded through a seal onto the control compartment (Der Tiefenapparatbehälter durch eine Membrandichtung am Torpedo ragte in die Apparatekammer hinein), The tetrahedral of the depth control spindle and the ventilating screw were located outside on top of the depth control container. Each time the torpedo was checked, the screw was adjusted in order to regulate the standard pressure in the container. At the bottom of the container was a rocker arm shaft which, together with the depth control mechanism, regulated the depth of the torpedo. For the designated depth there was a pre-tensioned spring, which regulated the air pressure and maintained the torpedo at the designated depth when it was fired. If the air pressure in the depth control container increased, the balance would be disturbed, and the torpedo would run deeper in order to re-establish the desired equilibrium by gaining increased water pressure. The compressed air that was used in powering the control mechanisms fed off into the control compartment, which meant that the air pressure tended to increase there. An exhaust valve was supposed to ensure that the increased pressure was released. Routine testing of this valve was not part of standard operating procedures . If excess air pressure seeped through the seal on the rocker arms into the depth control container, then the desired equilibrium would be disturbed and the torpedo would run deeper in order to equalize the increased air pressure."

(1) Wolves Without Teeth: The German Torpedo Crisis in World War Two by David Habersham Wright