The Quest Befriending Tachros got stuck on 2/2.  It want's me to gain affinity with Tachros and then gain the knowledge of Cron Castle.  Before i accepted this quest I had already gained the affinity and got the Cron Castle Knowledge, evidently you have to wait til you get this quest before you do the amity/knowledge gain otherwise you can't get it to update.  Also forfeit quest just says that it can't find quest data.  I tried logging in and out and that didn't fix it.  My OCD is kicking in hard and having a quest I can't complete and can't forfeit is driving me crazy.

I got stuck on this one for a day.

Turn on all your quests if you haven't and see if the chief offers you this again.

Mine popped after the last server maintenance and I had queste on.

The life skill quest menu is turning itself off at random making things like this happen.

Ya i went back to the chief and messed with him for a while after getting rid of all my quests and it didn't work, but then i quit and came back to the game 4 hours later and the chief offered the quest again even though i still had it in my quest log.   I got it and was able to immediately turn it back in.

Same Problem, have already the Cron Castle Knowledge and the quest won't validate, i'll will try to cancel/reset the quest

Finally, try to cancel quest a lot of times, each time i have an error message, quest won't disappear from my list but few minutes later Igor Bartali offered me the quest again and it was immediately complete.