Greetings,
I've done the quest about escorting healers out of the village, but i couldn't find them anywhere after that. Thinking that i needed to move on with other quests, i continued playing.
I've done a lot of quest after that, dealing with supply train of orcs, poisoning their water supply, going to Stormfist Castle and the whole Janus sequence, going to Verdists for Duke's Murder quests and so on. But i think i didn't get any main quest after that so i started wandering around and came across an area probably marked with plague.
Then i got three quests, each one requiring the potion for cure. That's where im stuck. When i looked up about other people's replies, i noticed they talk about a cutscene in "poor quarters" in the game, a cutscene including doctor elroth or something. But none of these happeden to me. I basicly ended up with 3 quests requiring me to cure 3 different people but there's no mention of the doctor or the cutscene yet. Is my game broken or am i missing something?

I THINK you have to visit his house but, it's been a while. I also recall the healers, at least some of them, are in the tavern.

The healers do indeed move to the Blue Boar Inn (rooms and tavern section).

You got these quests after entering the quarantine section, south of the poor quarters? There is a non-essential encounter where you see the doctor there, IIRC when approaching the main gate. Otherwise, check out his house, in south east Ars Magicana (hold Alt to see and interact with the door).

Thanks for the replies but i haven't seen a mention of "Ars Magicana" yet, there isn't a mention about the doctor either, or where he lives. And also there isn't a marker that shows Ars Magicana in my map, and i can't ask anyone about the doctor or where he lives so that maybe they can mark it on my map either. Am i doing something wrong or did i skipped a quest?

edit: i've just found out that i didn't discovered that part of the map before. I've reached the marketplace in Ars Magicana and encountered with the doctor, but he just said something general about the plague and left. There was no update on the quest log or anything. Then i tried to get into his house to get some info, but i could not lockpick the door. Am i supposed to do something else that i haven't realised yet?

Look for a key.

I found a key but it only opens the first door to his house. There's couple more doors leading to the interior but i can't lockpick them.

And my memory fails me on how I got past them. Seems like there might be a puzzle involving lamps/lights but it's a very foggy memory.

There is indeed a puzzle connected to the lights.
There is a window on the west wall of the back room, as well, where you can throw a teleporter stone through (screenshot).

Unfortunately i still don't have the teleporter stone since that imp took mine while entering the Stormfist castle and i still didn't completed that quest. I didn't know those two quest were connected like this. I think i should get that stones first. Thank you for your explanation Raze.

Rescue Zandalor (Rivertown sewers castle entrance) and let him bring you to a new area to get the teleporter stones back (after clearing the location).

The lanterns are the intended way to get into Dr Elrath's cellar. The teleporter pyramid trick is useful if you don't find everything you need in the cellar before you leave (after that, the house gets locked, and there is no way to open the front door again).

I entered the sewers from the market area entrance, cleared the first floor and a stairway led me into an interior of a building, i have no idea where i am but im guessing that must be the Stormfist castle, right? i encountered an executioner, killed him and now im stuck at a door with a guard standing next to it. I can't lockpick the door and i couldn't find a key either. Should i fight with the guard and then the key supposed to drop from him?
here's a screenshot

Again, due to time passing, I'm not absolutely sure but I don't believe you can get in that way. There is another room, a treasure room, that can be used to enter and continue the quest.

The other entrance may be marked on your map; if you have not found it yet, it is marked by a B on this map.

Thanks for the map screenshot, i rescued Zandalor and we ended up in the place of those council of seven, i also took the teleporter stone back from the imp there. Now what im thinking is if i continue to the quest gathering the council of seven and leave the Dr. Elrath mission aside to do it later, would it trigger any kind of quest-break or something like that? Or should i handle Elrath's house before?

I don't think it will break anything leaving Elrath's quest until later, but pretty much all you have left to do is check out his cellar and report on what you find.