I'm going to speculate this is a bug, because I can cast 1 or 2 signs at the beginning of the fight, but then I can't cast anymore. I have full endurance, and there aren't any effects on Geralt that would prevent him from casting.

What's going on? Anyone else?

Side note, I also run into trouble charging up a level 4 attack. If I switch to my other sword and then switch back, it works again, but I can't cast signs again until I re-load the save.

If the Koshchey is beating on you, you can't cast. Keep it off of you, by knocking it back with Aard or Igni.

I haven't seen anything like the problem with being generally unable to cast signs. Are you running any mods?

Thanks for the suggestion!

I wasn't using mods, and it wasn't when it was beating on me, I could be standing at the other end of the room and be unable to cast. I think it was a bug involving something to do with the interplay of Tawny Owl potion and something else--i found an old google cached thread where someone else had the same problem using Tawny Owl.

I stopped using the Tawny Owl, and I was able to use signs and beat the koshchey.

The koshchey fight was terrible for me. I never even got to see what it looked like. Going through the tunnels it was dark and I was out of Cat, so I used the other potion that lets you see hidden enemies instead, and so all I got to do was fight this bright white blob with long legs.

No, nondeplumage and the OP had it right. The mantis-like thing in the Prologue is a Frightener. The Koshchey is the monster you fight after navigating the tunnels under the Old Manor in Act V. The OP wouldn't have level 4 in anything if he were still in Act I.

I'm still not sure why taking Tawny Owl would prevent the OP from casting signs. Too much toxicity, maybe?

It's more or less solved, I beat the game...but I still don't know what the problem really was.

This google cache contains a conversation in which someone else couldn't cast signs vs koshchey

[url=http://webcache.googleusercontent.com/search?q=cache:u8O5zguVfOYJ:www.thewitcher.com/forum/index.php%3Ftopic%3D5688.0%3Bwap2+can%27t+cast+vs+koshchey&hl=en&gl=us&strip=1]http://webcache.googleusercontent.com/search?q=cache:u8O5zguVfOYJ:www.thewitcher.com/forum/index.php%3Ftopic%3D5688.0%3Bwap2+can%27t+cast+vs+koshchey&hl=en&gl=us&strip=1[/url]

I can come up with three possibilities.
1. A rare bug, regardless of whether it relates to the Tawny Owl
2. Something stemming from my hacky, idiosyncratic set-up (3d analyze + don't force min reqs).
3. Something completely obvious that I didn't notice.

I want to say it was either 1 or 2, because I was pretty confident in my understanding of the game's intended mechanics by that point. But who knows.

EDIT: Yes, it was in the Old Manor in Act 5 or so.

Have similar bug at fighting on the barricades. The funny thing is that sometimes realoading the save solves the problem (wtf?)

Having the same problem here! Noticed it a few times... might be due to Tawny Owl!