So I know this is a common bug but no matter what I do Peychar won’t show up. I slept in the infirmary until 12 am and then made my way to the barn where you are supposed to hide. But nobody shows up and strangely both doors that lead to Konrad are wide open, I don’t know why. Has anyone a solution for this?
I also heard that you shouldn’t let Peychar go peacefully because you will regret it later?
I wouldn’t call it a common bug. I waited for assassin in the bushes of the garden inbetween custodian house and church of Saint Cross. Not moving,in my “silent” outfit. He appeared 15-30min after midnight. Then I confronted him. Did it first time right. No need to reload save or wait another day. “Chameleon” stealth perk helps a lot in such situations and also your general stealth ability should be up to the task.
My stealth is good enough and I have the chameleon perk. But I guess it depends on the time and where you hide. I will try it the way you did but isn’t it buggy that the doors are open? Maybe that’s the reason he doesn’t show up? Because if that’s so it’s a bug and there’s nothing I can do. 
You can still close the door yourself 
I know but they are supposed to be locked, so that you can catch him while he is trying to break in, right?
Try to close them before and you will see. Maybe they will autolock.
I solved this bug by killing Peychar. I tried waiting for him, had my stealth at level 20 at that point, plus had the crouching chameleon perk. Waited all night for him to show up and got bored and went to the tavern. Found him sleeping in one of the rooms on the second floor and just stealth killed the bastard. I planned to do that anyway because in the previous playthrough he actually showed up, I confronted him and he backed off. After that there’s a cutscene of Keyser building the trebuchet and after it ends, Peychar tried to jump and kill him in the middle of the camp. Guards started pouring towards him and they killed him in the end. I’m told this only rarely happens though.
The same happens to me! He sleeps all the time and doesn’t go out of his room so I guess I’ll solve this the same way as you did. I had my doubts anyway because in many walkthrouhs they say it was a misunderstanding and that you should let him go. I knew he was a thug! 