This may be caused by an issue where the app is trying to load a dll specified in a configuration that may not be deployed in the bin directory, such as one of your custom DLLs. You can contact Support and ask them for the AssembliesPage.aspx, and have them help you use it, which can help you try to resolve this.
Has this issue resolved?, If so how did you resolve it. We are facing similar issue , we resolved this issue by deleting the temp files in the Microsoft .Net folder. But this issues pops up again after few days.