Most users may not have even realized there was a problem with the Marketplace recently, but for those users affected, the errors they are experiencing when trying to install certain apps are very frustrating. A portion of users have had issues with installing apps via the Marketplace over the last few days. Well, there’s both good news and bad news.
The good news is that Microsoft is aware of what’s causing the errors and believe that it is related only to devices which have been upgraded to Windows Phone 7.5 from an earlier version of the OS and only for certain apps. They believe that devices which shipped with version 7.5 should be unaffected. They also note that the issue stems from the certificates used to sign apps for the Marketplace.
The bad news is that Microsoft has temporarily disabled app publishing until they have issued a fix for the problem, which should be very soon.
Source: Windows Phone Blog