So at some point of time I noticed signs stopped working.
I press Q and see message "You cannot do that now." when no enemies are near or "This action is not allowed during combat" when in fight.

I obviously tried resetting key bindings, restarting the game, selecting different signs, etc.
Tried searching, seems like noone else here has the same issue. Thoughts?

I am having the same exact problem, dont know what to do, i have tried restarting doing all the same things and cant use signs at all at 30 hours into the game

I was unable to fix this and just started anew, creating a new save every few hours just in case.

Also stumbled on another bug in Novigrad, main quest related, when you're supposed to receive pop's mold antidote and diagram, and you get none. Cant' progress further without them.

I've just run in to the pop's mould antidote problem as well. It's not appearing in my inventory, nor is the manuscript listed among my alchemy recipes. I tried loading an earlier save but I didn't get the recipe a second time either.

About pops antidote - it's not a bug after all. It's placed in the consumables items slot. And since you have the potion, you don't need to craft it, thus it's not listed in alchemy tab. Counter-intuitive and badly designed, yes, but not a bug.

+1 I have the same bug. Did you find any solution?

Yeah, i have this bug too, but not at the exact same moment :
*SPOILER* i was still able to cast signs when you're fighting in the wood with Lambert, but when we came back to Kaer Morhen, i wasn't able to cast signs anymore, and now i'm fighting Imlerith and it's just impossible without sings...

Same thing happening to me. I had to turn the difficulty down to the easiest setting to defeat him.

So many bosses and not able to use signs is really hard work...

I found a solution on the CPR forum!
If you throw a Dimeritium Bomb at your feet and wait for it to expire, you can cast signs again!

I know this is an old thread, but this error just happened to me, and the d-bomb fix did not work; so adding this info in case anyone in the future gets this error and like me searches and gets sent to this thread;

Since the D-bomb error mentioned here did not work for me, I spent the last few days going through the codes/scripts to see if I could find the actual error in the game...

The error is in the geralt_stats.xml file

If you know how to edit the game's xml files, you need to go to all of the strings of code for the signs (20 of them, you need to do the skills upgrade versions as well)

for every line that says "stamina_cost" the number after it reads "min 30 max 30"; but it should read "min 3 max 3"; changing all the 30's to 3's fixes the error.

Hope this helps anyone who runs into this in the future and d-bombs don't work for you.