I posted the links recently to download Update 11 for Revit 2015, over here. Unfortunately, this update causes problems with Revit Server.
While there is no official hotfix (yet), you can still have Update 11 and Revit Server if you follow one of the 3 methods at this link to fix your Revit install:
Revit 2015: UR11 breaks Revit Server | Revit Products | Autodesk Knowledge Network
Method 1 involves modifying Autodesk.RevitServer.Social.dll, Method 2 involves modifying Revit.exe.config, and Method 3 recommends not installing Update 11 (?)
Check the main article for the detailed steps.
