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If you upgrade your installation of Telerik Report Server to Q1 2016, chances are that you will see this message right after finishing the installation and opening the Report Server Manager. The reason is that two of the assemblies that Report Server uses for authentication purposes are not copied into the \bin folder of Report Server by the MSI installer. These assemblies are Microsoft.AspNet.Identity.Owin.dll and Microsoft.AspNet.Identity.Core.dll.
The installer itself doesn’t copy the assemblies, because their versioning changed since our last release. In the previous release of Report Server Q3 2015, both the assemblies had version 2.2.40217.0. However, the newer version of these assemblies is 220.127.116.11403, which is considered smaller (older) than the previous one by the installer.
The easiest fix is to run the installer again and choose the Repair option from the Customization page. The installer will copy the missing assemblies into the \bin folder and the Report Server should start successfully.