According to KTB Adm i.d. Niederlande:

At the time of the sinking H 32 was anchored off Vlissingen (Flushing) together with the H 31 and Vp 1512 and Vp 1510. There was a heavy fog at the time. Aboard the Vorpostenbooten the explosion heard but not seen. The H 32 had a 48 man crew (including 9 Flaksoldaten) of which 7 man were missing.
On the beach of Dishoek (near Vlissingen) an undamaged torpedo was found together with the remains of a second one.

In the book “Coastal Forces at War” (by David Jefferson) there is the following story:

In December, Jermain had another opportunity to use depth-charges at close quarters. With his SO Lt-Cdr Cole and Lt P.M. Corsar, the three boats from the 4th MTB Flotilla were on patrol off Flushing and the estuary to the River ScheIde. Lt-Cdr Cole's boat had to return to Felixstowe with engine problems. Still under power on their main engines, the other two boats suddenly came on a large convoy at anchor, further out than was expected. The convoy escorts, alerted by the noise of the MTBs' engines, immediately opened fire. Lt Jermain lost contact with the other MTB, which was hit several times, sustaining damage to her steering and torpedo firing equipment. MTB 31 was now on her own. Twice she attempted to torpedo one of the armed escorts, but on both occasions Jermain bad trouble with the firing mechanism and the torpedoes failed to reach the target. With no torpedoes, he selected the largest ship at anchor, and made two runs withrb bis depth-charges. On the second of these, MTB 31 actually scraped the bows of the German ship. Both depth- charges exploded beneath the target, and although Jermain did not stay to see the results of his attack, she was believed to have been a vessei of over 6,000 tons.

This story fits remarkebly well the KTB Adm. id. Niederlande.
With caution one can draw the conclusion that Birkenfels was sunk by depth charges from MTB 31 while she anchored of Vlissingen. Even for the misfired torpedoes is accounted.

I do not have access to the KTB of the 15 Vp-Flottile so I cannot check whether the VP 1510 and 1512 actually detected the MTB’s and fired on them.

Who knows the name of they other two MTB’s, I expect one of these is the MTB 32.

I like to hear your reactions.

Greetings from Holland, Hans