The Winter Ice in the Baltic would be forming in this period (it usually begins from October to November, 1941 was a cold year), I would guess that's changing calculations in Sweden? Oct 1st 1943 was both the date of Allied demands for Sweden to halt the Lejdtrafiken with Germany (implying a perception that Sweden would be relatively safe from German retaliation by that time?) & the day of Hitlers order for the Danish Jews to be rounded up (implying a perception that the window of opportunity to threaten Sweden into non-resistance to that was rapidly closing?).
The winter of '41-'42 was exceptionally severe, to the extent that by February '42 the entire Baltic Sea was frozen - a very rare occurrence. However, seeing as the Gulf of Finland would be entirely frozen only in December, I'd say that the Baltic waters south of Gotland could be used for naval warfare and transporting troops and materials at the very least until late November '41, entirely or partially. On the other hand, I'd definitely say that anything north of Gotland on both the Swedish and Finnish sea areas would be realistically unreachable for the Germans by ship between mid-December '41 and late April '42.
Unless assisted by Union or Soviet icebreakers, of course. But then even with icebreakers, there are definite limits to what you can do.
(Incidentally, while checking sources on this, I came upon the claim that the Germans IOTL thought the winter so severe that they suggested running a narrow-track railway connection from the Estonian coast to Finland over the Gulf of Finland sea ice at this time. The idea was rejected by the Finnish sea ice expert Dr. Risto Jurva as too risky, but it does give us some inkling as to how Berlin saw the situation. IOTL, in fact, Finnish icebreakers were sent south to help the Germans in March and early April. As late as on April 8th '42, German cargo ships needed the help of the icebreaker
Sisu around the port of Libau!
Another example of the severe winter is the fact that the Finns IOTL recaptured the island of Suursaari [Hogland] on the Gulf of Finland via an attack across the sea ice as late as the last week of March '42. The operation consisted of 3500 men, most transported to the area by trucks and horses on roads built on the sea ice.)