Basically I got Shadow Magic for myself and my friend.
We both DLed it from GOG, no extra patches, no mods, no nothing.

When we try to connect via about anything... "Incompatible resources"

Maybe you run different versions of the game? That would be the case if you downloaded the game earlier than your friend, so you may have an earlier version of AoW while your friend has the latest.

Make sure you both have the same version & patch.

We downloaded it with about 1h interval 3 days ago.
Obviously we also compared versions, and they matched to the symbol.

Nope, still same issue.
Even more so. My other friend can connect to me just fine from his Steam version but the first guy... nope. Seems like it's an issue related to his PC but he has ports forwarded, firewall is also set to let game through, etc.

Sounds like it's time for some PBEM (Play by email) using the File wrapper from Davespice. :)

Networking old games is always a bit touch and go. When my friends and I play together, we often have multiple games concurrently. That way you can play a turn in Game A while your friend is playing a turn in Game B. As long as you play different starting Wizards, races, and/or maps, it doesn't get too confusing.