That happened to me as well, although only in one game. The next game he could attack as normal again.

I noticed this as well. I think it's because he has 0.5 movement points in battle, which effectively glues him in place. I didn't test luring an enemy near him to test if he could attack or not.

I encountered this as well.  It seems that movement has been switched with combat speed in tactical battles.  While Janusk has a movement speed of 2, he only has a combat speed of 0.5.  When in tactical battles, however, this is switched, which effectively roots Janusk on the spot. I read in another post that these two values have been linked. As is evident from gameplay, this is extremely detrimental and not reflective of a character`s actual stats. These stats need to be delinked, if indeed this is the case. Otherwise, the switch of stats is a bug and needs to be rectified.