I set up a new scenarios with same scenario options and just tried to level up.
In the new save file everything works, so the bug is specific to one safe file.

At first I thought "Great, exact line of the error and the problem, probably an easy fix ". But that line of code should not really cause any problems....unless you deleted removed Volkare's skill bag. I tried, and that's not easy to do. It would have to be done intentionally, as I couldn't find a way to do it accidentally. So I'm a little stumped. without your save file I can't really investigate it further. While I can write a check to see if it's missing, your game is still not correct as it will continue without a dummy skill. I'd rather figure out why it's missing (or registered the wrong skill bag) and fix that. Don't suppose you kept you save?

Wow, that's a fast reply. Thanks!
I do have a safe file of it. Though it was quite early on in the scenario, so I just started a new one, in which everything works fine.
So I am good now. If you are still interested in the safe file just for bug fixing, I can share it with you. Just tell me how to. (share the file / host a session together? )

I was fast, then got distracted. Here’s a folder you can put a save file in…..if you still have it.

https://1drv.ms/f/c/18c549927b9fc512/EhLFn3uSScUggBihLQAAAAABGONTfOl7qkKcVUbzU4j9Cg

I had the same issue than Raketo, i droped my save file in your link if this can help you ;)

Yep, when you look at Volkare's play area the problem becomes obvious :-)
I've fixed Volkare to pick a random skill from the Mage Knights properly. Only Ymirgh was filtered out before the fix.
Thanks guys.

Thanks for your work!

Hey it's me again! We had a strange bug, happened two times already, so i droped my save file in your link again!
It's when Volkare try to attack our city, the script for the coop defense bug. We can't split monsters against the players. We found that if Tovac isn't in the city, the script work, so maybe a bug with Tovac?

Thanks for the report. While tricky to track down I believe I strangled the little gremlin. I put a fixed copy of your save back in that folder, if your were tempted to finish it. Looked like you guys blitzed to the city somehow!? Volkare had a lot of orange arrows.

Not really blitzed to the city, but Volkare had a VERY SLOW start if you look at his discard pile (wound, red, wound, blue, green, wound, red, wound, wound, wound, white, red, green, wound, wound and red) so he waited a lot at the start!
Thanks again for the fix, we will try a new game to see if the bug is still here!