Hi there.

I've finished the quest Reason of State and decided to help Roche. I have finished the quest and the game marked it as completed in the journal. Unfortunately, after starting the final main quest each time I'm being informed that I've failed to complete the quest Reason of State. Any sugestions how to fix it?

Thanks in advance for help : )

I don't know.

Open the journal and check the failed quest. Which stage did you fail on?

Exactly the same happened to me... It is pretty annoying, as this side quest affects the ending of the main story :(

Had you try to complete game, to see which conditions were used for final decisions?
Some quests could be just moved to failed category. I don't have this, but have some other. This quest, however, gave me enough problems, because three witch hunters refused to join the fight, and were standing like ordinary bystanders, yelling and waving hands like they were "adoring fans". Scripts didn't work, so it was impossible to complete this mission. :D

In the journal it is mentioned that I've decided to help Roche and that it will never be know why Geralt have never brought the matters to the end. It's a loose translation, sadly my English isn't very well.

I didn't complete the main quest. I'm a perfectionist and it's killing me that I've failed a quest : ) I know that it's stupid but I'm trying to find some solution before I will continue with the main questline.

I had the exact same bug with this quest. It didn't affect the ending whatsoever, though.

Another person had a problem with a quest randomly failing and fixed it by removing the files that enable the dev console. Now that I think about it, I only had this problem on a second playthrough when I started playing around with said console, so removing the files that enable it (assuming you're using it) might solve the problem.

SPOILER ALERT
I've killed Radovid the V and decided to help Roche, Ves and Talar so I've killed Dijkstra and his men. Roche said something about a courier that is going to be assassined by Dijkstra men and that they need to stop it. He also said to check on them some day in Vizima. After that talk the game noticed me about completing the quest. I've tried to find them in Vizima but was unable to so I've moved on to the main questline.

I didn't enable dev console, no idea how to do that neither.

Ok, I found this, which suggests this is a known bug, but the quest will actually be treated as completed in the ending dialogue.

It looks like that the newest patch 1.06 fixed this bug. Yay!

I have a theory on this one. See there are two possible choices, Roche or Dykstra. When picking one the other naturally never gets picked. The bug most likely is the other choice remained as open and that is what failed.

I do know that the game enacts whatever you picked so in rality it doesn't actually fail.

Ok same for me, but in the Final Scene Raduvid is Dead, so it seems to be a Bug, but it is not impacting the Game.
But schouldn`t alle the Witch Hunters in Novigrad be gone with Raduvids Death? Or is their just no big change to the World after the final Scene?

Same here, with patch 1.08...

Same as above, I just beat the game again, with 1.08, having already completed Reason of State (as in, I got the "Quest Complete" message, with the female chorus and everything). I start up the final mission... quest failed for some reason. I sided with Roche, and at least the ending correctly gave me the right scene for the fate of Temeria.

I opened up the "failed" quest entry, and none of the objectives had a red X. Not to mention, as stated above, the game clearly told me before that I completed the quest just fine.