Thank you Vadim, that was clearly a dumb mistake by me… now everything works.
Just another quick question so that I completely understand how filtering works.
From the experiments I did, the only way I found to redirect some packets and let everything else pass is to use MSTCP_FLAG_TUNNEL for the adapter mode and then specify 2 filters: the first with the action FILTER_PACKET_REDIRECT that intercepts the packets I’m interested in, and then a second filter with action FILTER_PACKET_PASS to let everything else pass through. Am I correct? Or is there a way for example to set the adapter mode in something like “let everything pass” and then use a single filter with the action FILTER_PACKET_REDIRECT?