So there was no need to post everything after all because I was able to track down the issue.

If anyone else hits this problem, the thing is that when spending p2sh-locked utxo, one needs to put into the input's scriptPubKey (when signing) the full redeem script, not the 23 bytes long (a9 + 14 + hash + 87) scriptPubKey that was used to lock that output in the first place (like it's the case with normal p2pkh transactions).

Anyways thanks a lot for all the help