I've been using Plex for about 2 years now and have never had the problem I have now. Plex stopped working two day ago for me on my Apple TV. None of the posters come up nor do the movies or TV shows play. But oddly Plex only works with about 3 mp4 movies and wont play the rest of my library which is around 50 movies and 30 episodes of TV shows and most of those are mp4. Ive never had a format problem whether the movie is a divx or avi or whatever format.
I updated the Plex on my Apple TV and MacBook Pro today and when I launch Plex now it acts real slow on my Apple TV now which isnt a huge problem just seeing this has to do anything with it.
I got on the local wifi with my iPhone and Plex works on my iPhone and all the movies and poster play and come up fine.

I use an external hard drive with my MacBook Pro and store all the movies and TV shows on the external and sync it with Plex Media Manager. I tried just syncing the movie folder on the internal hard drive but I still get the same problem.
I can use AirPlay and stream music through iTunes from my MacBook Pro to the Apple TV

Plex on my Apple TV sees my computer, sees all my movies and TV shows, but wont play them or come up with a poster but it does come up with the info on the movie or TV show.
When Plex does not play the movie or TV show it says "An error occurred loading this content. Try again later."

No changes have been made to the Apple TV or my computer nor the movies. I go to the folder where the movies are saved and they play fine on my computer and like I said when I connected to the same network with my iPhone it works fine when i play it through Plex.

Ive tried:
- Re-installing Plex on my Apple TV and MacBook Pro 3 times
- Deleted all my movie sections and re did them with in Plex Media Manager
- Re-started the Apple TV a 4 times along with my computer
- Plugged the Apple TV directly to my MacBook with an ethernet cable
Non of it works.

**** I plugged my external hard drive (with all the movies and TV shows) to my MacBook Air and downloaded Plex and set it up just like its set up with my MacBook Pro and it works fine, the posters come up, Plex is not slow anymore and it plays every single movie and TV show just like normal.

What could be the problem? I totally un-installed Plex from my MacBook Pro with an App called CleanApp which deletes every little file from an app you choose to uninstall even the settings were deleted.

Im running Mountain lion (10.8.2) on my MacBook Pro which is 2.3 GHz Intel Core i7 with 8 GBs of memory with Plex Media Server .9.6.9 (most recent version) -- Same thing with my MacBook Air
and IOS 5.0.2 on the Apple TV which is the most current that is able to be jailbroken with Plex .10.107 (most recent version)