The message - "you were not found in active user list - session may have timed out or logged off" appears in multiple Web Viewer window panes, and no one can access any folders.
MS Windows 2008/2012 Server
Web Viewer 11.5
What may be behind the problem is an incompatible IIS, DefaultAppPool definition for the Web Viewer Web Site. In the IIS Manager, Advanced Settings for the DefaultAppPool, under Process Model, the Maximum Worker Processes (formerly Web Garden in older IIS versions) should be set to 1. If it is set to anything more than 1 - say 5 or 6, it will cause each element in Web Viewer panels to be treated as a separate request or session, resulting in you seeing the same "you were not found in active user list" error echoed in several panels of the Web Viewer session window. You would be able confirm if this is the problem by opening the Windows Task Manager on the server, noting whether multiple W3WP.exe tasks were opened. There should be but 1.
Change Maximum Worker Processes to '1' for the DefaultAppPool and recycle IIS to resolve the problem.