i havent found whats actually caused it but i experienced it a few times in the meantime.

Sometimes the Main Char isnt able to do anything while combat. All greyed out (or darkened). I cant even move its like the AI moves currently the whole Char is disabled there isnt even a "not possible" message or something like this.
Only thing i actually can do is to end the round. Then the Char moves to the position i clicked last, every other char works as usual.
If the fight ends and the Group is still alive i can move with the main char but cant activate anything. No chests, no Spells nothing.

What helps is to reload the Game but you shouldnt face a hard fight with this or you have to fight it without one fighter....

Dunno if this has something to do with it but that char is a warlock. I played a Mage before and never had this issue. Might be related maybe not.