I'm in the fight with Skule. He has an npc on his side with the Name Hafstein Grankelsson. Everytime its his turn the fight stops in combat mode, nothing happens and I have to alt+f4 the game.

Try these console commands

flushstack
changeturn

the - key (between 0 and =) opens the console

What do these commands do, and is there an up-to-date list of known commands?

I believe flushstack clears whatever command is running and changeturn ends the the current turn and win will end the combat with a victory if those don't work.

There are quite a few console commands on the Steam boards but dunno if anyone has collected them all in one place