Beepster
Now that I've made you sufficiently parnoid (sorry... lol) this is probably just some kind of confusion on youtube's part.
Really you disputed it. They'll probably get a real human involved to check it out and all will be well. Do NOT contact "Party X" though just in case they are skeevy fudgemonkeys out to screw you. As soon as you make direct contact it kind of opens the floodgates to more harassment and litigeous bullpuckey.
All will be well I'm sure.
The cost of getting a real human involved is probably prohibitive for all concerned. If you read everything
here, you will see that Google is phrasing their appeal process in such a way that it will frighten (warn?) most readers into simply accepting the ad or removing the video. Your appeal will give Party X all they need to have a robot generate a cease and desist letter complete with realistic machine signature and the name of a real lawyer. But even without such a costly mailing, the reputed copyright owner can wait thirty days then just tell Google they still think you are a pirate at near zero cost. Google is not involved at all here and pays no cost, it is you vs the complainant. The complainant does not have to offer any evidence whatsoever to either you or Google that their position is correct, a simple statement (again machine generated) that a given work has been infringed is all they will ever have to provide. You can then appeal the rejection, and after another 30 days the claimant must either release their claim, or they must issue a takedown notice. A takedown notice can be machine initiated with the information they have from the appeal process and there is no filing fee or other apparent significant disincentive. If they do issue a takedown notice, again without any proof or evidence or evaluation of the merits by Google, you account will receive a "copyright strike." Google helpfully "warns" you that:
"Because an appeal may initiate the takedown notification and counter notification process, the contact information you provide in the appeal form will be shared with the copyright owner, who may eventually choose to take legal action against you."
If the takedown notice is issued you are in copyright law territory, and you have a statutory appeal. Google takes that takedown appeal as your only defense against the "copyright strike" it has issued without any hearing of the facts itself. It is not clear if the strike is suspended during the appeal process, but it appears that the post will be taken down pending appeal. The complainant may issue a retraction at their discretion, which will apparently wipe the strike and reinstate the posting, but there is no reason for them to do so. If the complainant simply lets things slide without further affirmative action on their part, the strike apparently remains. And again a warning:
"Please note that when we forward the counter notice, it will include the full text of the counter notice, including any personal information you provide. The claimant may use this information to file a lawsuit against you in order to keep the content from being restored to YouTube." In fact the same information can be obtained by subpoena regardless of your filing an appeal.
The statute says that anyone who files a takedown notice must include "A statement that the complaining party has a good faith belief that use of the material in the manner complained of is not authorized by the copyright owner, its agent, or the law." A good faith belief is a very elastic concept, and proof of actual infringement typically requires a trip to court, so unless the claim is clearly fraudulently made, or absurd on the face of it, the complainant has little to fear about doing so. If you file a formal statutory Counter Notification Letter, the law requires YouTube to restore your posting unless the complainant actually files suit for copyright infringement within 14 days. That step is very likely to be more costly than it is worth to the complainant. If he does not file suit, you should be able to get your posting put back up, but it is not clear that YouTube will retract the "copyright strike" unless they get a formal retraction from the complainant. If he does file suit, the cost of simply answering the suit will almost certainly be more than you could hope to gain by the posting, and if you lose a suit, with statutory damages it could cost you several orders of magnitude more than the benefit of the posting.