If you are having problems connecting to CoRe, the first thing you should try is resetting the IIS. The easiest way to do this is to open a command prompt on your server and type in "iisreset".

Command prompt can typically be opened by pressing the WINDOWS key and typing "cmd" into the search field. This is recommended to be opened in Administrative Mode which can be done by holding SHIFT+CTRL and then clicking on the cmd.exe process. 

If that does not work you can open the IIS manager and click the restart button on the right side of the window. 

The IIS Manager can be opened on servers by navigating to the Control Panel and clicking System and Security > Administrative Tools, then double clicking Internet Information Services (IIS).

This resolution should also help in many cases where CoRe is not able to upload photos or users are being disconnected from the system frequently.