I followed the main quest up until "A Balde in the Dark". After that I started to do side quests like putting an end to the Stormcloak rebellion in the name of the emperor, becoming the newest member of the Dark Brotherhood and restoring the Thieves Guild to its former glory. At this point in the game I decide to continue with the main quest.

After starting "Diplomatic Immunity" I gave some of my gear to Malborn, killed a dragon outside of Solitude, talked to Delphine and got on the cart. However, when I arrived to the embassy, Razelan is the only guest at the party. No Jarl Idgrod, Erikur or Ondolemar. To make matters worse, I can't give Razelan the brandy that you get from Malborn. So I can't start a distraction and continue with the quest.

Do you have any ideas on how to solve this problem?

That's very strange. Is the maid there? She offers brandy as well, as I recall.

I guess Elenwen isn't as popular/important as she thought she was. It's weird 'though.

Anyway, only option I see is reloading (if the brandy from the maid can't be given as well). Worked for me, when the wood elf got stuck before opening the door to escape the party.

The maid is there and she offers the brandy too. I talk to Razelan with the dialog "Here have a drink on me" and the inventory menu pops-up but there is nothing there to give. If I then check my inventory I have both the brandy and a couple of bottles of wine.

The only people inside the embassy are Malborn, Razelan, Elewen, Brelas and a couple of Thalmor soldiers.

I have also tried to reload the game a couple of times but that doesn't change anything.

Have you talked to Malborn yet? Sometimes you have to talk to him before you can cause the distraction.

Yes, talking to Malborn is the first thing I do. I've done this quest before on another char and that time everything was normal.

I had an issue like this myself. Except I hadn't done any of those questlines. All I had done was the Companions, and the only people there (outside of the people necesarry for the quest to continue) were Erikur and Maven Black-Briar.

What a wonderful shin-dig that would have been.

Did you complete the Civil War questline? Someone mentioned in another thread that you can't go back to the Thalmor Embassy after the Civil War - or can't go back to kill everyone in it at least.

have a drink on me should be enough. I did not get an inventory memo. Assuming you click on have a drink, return to Malborn. He should guide you through the back doors. Check the wiki that you have done the right quests before arriving at the embassy. And say hello to the host when you arrive, the frosty Thalmor [censored].

I also didn't get an inventory screen.

I can click on "have a drink on me" but according to the inventory screen I don't have a drink to give. Even tough I got the brandy.

Reloaded the game to a point way before I completed "Civil War" but that didn't help either.

I'm also having this problem on a character that I recently started. It seems like I might be a bug that was introduced in the 1.4.21 patch, as I can't find people that were having a problem with it before then. My current theory is that whatever fix Bethesda implemented for the issue with the Stone of Barenziah ending up borking something else. It's obviously contingent upon some other quest/NPC as not many people seem to be having the issue.

So, here's my situation:
--no guards and no guests in the Embassy
--only Malborn, Brelas, Elenwen and the Redguard are present
--I can buy the wine from Malborn, but the Redguard won't sit down and give me the proper dialogue, so I can't give it to him and create the distraction.
--As a result, Malborn won't unlock the door and I can't proceed with the quest. I've attempted to complete the distraction phase with the console (setstage MQ210 50), but it will not advance.
--This obviously breaks the main question chain of the game.

Other imporant details:
--I killed Astrid and disbanded the DBH when being trapped in the Abandoned Shack after killing Grelod
--I just went with Farkas to get the fragment of Wuuthrad and got my Skyforge weapon; not a werewolf yet
--I have not even enlisted in the Stormcloaks or Imperial Army; 0 progress on the Civil War chain
--I have not started the Thieves Guild chain; Brynolf is trying to get me to, though!
--I have not started Blood on the Ice; don't think this would matter, but considering all the problems involved in that chain...
--I wiped out the Thalmor at Northwatch Keep when rescuing the Thorald Gray-Mane

Note: There is now record of this glitch on UESP and TEX Nexus. I've also reported it for the Unofficial Skyrim Patch.

Wonderful; we not only have to deal with existing glitches and broken quests, but brand new glitches that affect older glitches, and they call it a "patch."
The fun thing is that we're all affected differently, in out own little glitchy ways!

I'm pretty sure most of these were the conditions under which I did Diplomatic Immunity on my 1st character (except I was past the Werewolf portion of the Companions line). I had finished Blood on the Ice though. Damn, this is F-ed because this is MQ stuff.

I had the same thing happen to me. All the Thalmor people were at the party but no guests except the drunk. However the drunk would not sit on the bench he would only stand and not engage in conversation. I can get the drink but I can not give it to the drunk. Malborn will tell me he is ready at the door but this is as far as it gets.

No amount of reloading or exiting and reentering will get the guests to appear or the drunk to accept the drink. setstage commands did not work either. Finally I just went up to the door and unlocked it with console command. I then did the same thing with the second locked door. As soon as I opened that second door the quest reset and updated to the proper stage. I got my equipment from the chest and when I walked through the door all the Thalmor immediately turned hostile. I do not know if this is suppose to be or a glitch. I could then either fight them all or try to hide. I went invisible and hid till the settled down and when they did they were back to normal and I could sneak again.

If you are on PC and at this bugged stage in the MQ you may try to "unlock" the 2 doors you need to move through and that may reset your quest and let you advance.