Hallo,
A difficult task Platon

It looks like the attack 20.1.45 wasn't observed, as I can't find it in Ultra nor in the few surviving KTBs from the area.
From KTB 5. M-Fl and 17. UJ-Fl the only convoy I can see fits is southbound St-779-Ks (sailed 16.00 from Stavanger. at Skarvöy 00.00, and arrived Egersund 02.00 as they belived there were surface ships in the area). This convoy consisted of: Erna, Krückau, Storefjeld and Hillevaag. Escorts: UJ-1702, UJ-1706, UJ-1709 and M-251.
21.3.45 (from ultra) the UJ-1432 reported 2 torpedoes exploding 13.31 at Signalstelle "Apenvaagsoten"(must be a strange spelling). This fits very well with the attack, and UJ-1432 was in the area (Folla between Trondheim and Rörvik / Namsos).
20.3.45 UJ-1432 reported D. Leba damaged by grounding off Melöy, but that she continued with her towards Rörvik. 20.3.45 Leba sailed from Bodö (for Oplö off Namsos), Gordias (Bodö - Sandnessjöen) and Torridal (Bodö - Trondheim) were due for sailing. In addition V-6402 escorted U-325 in this area 21.3.45.
Unfortunately no telegrams in ultra gives the whole convoy status for Nordküste area 21.3.45.
My theory is that the Venturer attacked the UJ-1432 sailing alone from Rörvik, or the Torridal (should have had escort)
23.3.45 UJ-1430 attacked a submarine in AF6488, but handed the hunt over to V-6409 which attacked the submarine in pos. 64.38'N 10.48'E
Best regards,
Erling