When i start the quest, I have to go to the bathhouse. After a brief talk with the guys assassins storm the place and the objective says "defeat the assassins".

Now ... there were 2 level -1 assassins, that didn't move, just stood there and were unkillable. They still were after reload and restart. After a few tries I thought to download all free DLCs. Maybe it got patched there sometimes.
For a second there I got really happy. Since there were no more glitchy "Whoreson's Henchmen". Not that obviously at least. Now there remains one lvl 11 Henchman whom I still can't kill. The only difference is the lvl and that he fights back.

During the fight, someone screams out "We need one alive!" - so i thought to capture him with the upgraded Axii. No chance. The objective is to defeat all of them, so i probably have to kill him, which doesn't work.
I tried all Signs out with different combinations ... but nothing works. So. Does someone have an idea how i could get past this scene? Anyone had something like that already?

Yes, I see that too, with 1.11 patch. Sometimes there are two unkillable assassins, one of them doesn't move at all, and doesn't react to hits or signs, sometimes just one which can be stunned or hit, but cannot be killed.

UPDATE: I tried the Debug Console from Nexus mods (http://www.nexusmods.com/witcher3/mods/143/) and killall command, but I still cannot kill him.

I've got the same issue. Tried that part 4 times and always there are 2 unhittable (-1) assassins...

I have same problem. Two NPC with -1 and can't be killed. Thanks this bug I can continue with game :(.

Sorry, don't have a solution, just want to add that I, too have this problem. And this is with after downloading the new patch. 1.12

I have the problem too, any fix yet?

Patch 1.12.1 for me, and exactly the same problem, two thugs standing around that cannot be killed. May reload earlier save and try to avoid the quest

I have the same problem. I also noticed, that Djikstra stands there without doing anything, so I think, he is bugged too.
Patch 1.12.1

I tried Destination : Skellige and the same thing happens when the pirates attack the boat, a couple just stand there with no HP left. This means that I also can't progress here, thus cannot progress anywhere in the main quest.

Still patch 1.12.1. Is there an official way to report this to CDPR?

I just verified my install a couple of times through GOG Galaxy and now it appears to be working correctly..

Having the same issue during the pirate fight. Four inactive NPC. Since I can't kill them I can't continue the storyline. How did you validate with GOG? I reloaded the game from disks to no avail.

On the Witcher 3 page on GOG Galaxy, next to the green play button there is a more button. Click that, then go to manage - verify/repair and it should download a small (25mb or so) file

Thank you, verifying files worked for me.