In pc version, patch 1.10, game is remains locked in combat mode near the campfire in Fool's Gold quest, if you have chosen to fight bandits who roasted a pig. Bandits have -1 life immediately after the dialog is ended and cannot be killed.

Yeah, it's a bug with latest patch, as reported by many users here.

https://www.reddit.com/r/witcher/comments/3o3j0m/110_update_bugs_and_problems

We are all waiting for a hotfix, so just do some other quest before the fix is released.
You can try and verify your game via GogGalaxy, but it has a 50/50 chance of working for you.

I have reported it to cdpr support with save file attached and got something like "your game was modded or there were changes done to game's files". Also verification of installation haven't helped.

Very funny, but they have normal amount of hp before 1.10/HoS.

Seems fixed in 1.11.

I have another serious but similar issue with this quest.

When you end the conversation with the initial group of 4 bandits, you only get a chance to kill 3 of them before the cutscene takes over. The last bandit wanders over to the fire again and you proceed with the quest.
However when you return to herd the pigs to the cave, if the last bandit sees you he immediately becomes hostile and unkillable and the quest is rendered broken. The only way to get past this is to ensure you are unseen or cheat with the console and kill him that way.
This has happened twice to me now with my initial 1.09 version and 1.11.

Seems a bit different bug. I would suggest to report it to CDPR directly with saves attached. It surely more useful than report it to gog support.

Normally I would, but for some unknown reason I cannot create an account or use my GOG account to login to their forum to lodge the issue. I can't contact them to find out why and I figured GOG's forum was the next best thing.

From my searches around the NET I seem to be the only one that has seen this, plenty of entries describing the issue of the initial post, but nothing like mine.
Posted it on Reddit too in the hopes someone else may have seen the same thing and they might be able to post an official report.

I will try again with another browser later, just to be sure.

EDIT:
Figured out my login issues, 3rd party cookies settings! As soon as I set an exception in Chrome for all the GOG sites, my login details were used correctly.

Forum is another thing. I suggested the CDPR issue submission form http://en.cdprojektred.com/contact-support/ .

Done.

Contacting the support team has indeed resolved this issue.

The instructions to verify the game via GOG Galaxy was offered and although GOG Galaxy reported no changes were necessary or any additional files needing to be installed, this quest is now working.

Not exactly sure now what was changed or what happened but at any rate the unkillable bandit does not appear.

Well looks like this issue is still around i have both the -1 unkillable and just standing there and i can kill them and they still stand there and quest does not continue.