I have completed the main quest (level 47, reputation 91) but...

While the "common people" cannot find the right words for talking with me (I don't like that but, well, it's their dialog items), the questgivers, like the Wise Woman in Urshilaku Camp, the jester in Tel Fyr and some others are very disappointed with me: "Why aren't you following the Nerevarine path? Go to archcanon, he is waiting on you desperately...", and so on. Only Vivec is aware of my finishing of the main quest and he is sending me to cleanse the Vvardenfell from blighted monsters, etc.

But, of course, I have visited archcanon long ago. I have spoken with Vivec and got Wraithguard from him (and then I tried to inform the archcanon about the interview but he didn't seem to be interested in it). I have killed all Dagoth Ur's vampires and then Dagoth Ur himself. What's wrong? Did I forget to do something? Or, is it the dialog database, which is wrong there and which is letting the "important people" to suffer from Alzheimer diesase?

I have the same problem, so it's unlikely to be anything you've done wrong. Got to be selective amnesia methinks [img]smile.gif[/img]

It maybe the dialog problem. go talk to normal dunmer and see whether they acknowledge what you did.

There is no reason to return to these people after you finish their quests, they are only able to repete the last script in their diolog.txt that would send you to complete what is needed to become Nerevarine. I can't see any reason why you would want to talk to any of these people again unless you just like having your boot kissed.
There are alot of other things you could be doing besides going back for unneeded conversation with people from past quests. Just go back to exploring Morrowind and kill off all the ASH heads and demons that are still in caves all over the Island not to mention all the treasures still to be found.

In my games, dozens of NPCs say incorrect things. They're just little bugs in the dialogue, I'm surprised that you haven't encountered many before. It's distracting when you come across things that don't gel with the current events. In such cases, just blame Sheogorath. [img]smile.gif[/img]