Okay, I have no idea why this is happening.

I start the quest, Diplomatic Immunity and for some reason there are no guests at the embassy.
I do have several mods running, however even though I turned off almost all of them to try to figure out what is happening, I cannot seem to be able to fix this.
The only people there are Elenwen, Razelan and Malborn. Also there are some guards and a serving girl.

I have succesfully played through it before and remember using Razelan as a distraction by getting him a drink. However now he doesn't respond to me, even when I have a drink on me, although he does seem to take wine, but that doesn't cause him to cause a distraction.

Does anyone know what I can do about it?

I could use the console to try and prid all the guests that are supposed to be there and moveto player them to the embassy, but that would be an awful lot of work and I have no idea which characters are supposed to show up.

I am also pretty far in the game, on the side of the empire and taken a few of the holds. So several jarls are already changed.

Nobody can help me?

What mods have you got?
If you are on the empires sides the Jarl of Morthal is there.
Thane in Solitude Erikur should be there.
In my game the would be Jarl of Riften Maven Black-Briar should be there to.
And that drunk redgurad should be there as well.

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I will move this now.

Just note that even if you removed the mods, your game may be affected by those removed mods. Try loading a save from before you even took the quest, and then 'clean' your save by removing all mods and saving. Also, check the removed mods' readmes for uninstallation instructions. Many mods add scripts which may still function even though the mod is unchecked in the load order.

Edit: If you are relying on autosaves only, you can go to the folder My Documents -> My games -> Skyrim -> Saves and find backups of the autosaves. Just remove the existing autosave and rename the extension of an older autosave. While this probably won't take you far enough back, it may help.