When you try to create a Database Availability Group (DAG) with the Exchange 2013 web-GUI, the interface throws the following error "error Access Denied The operation wasn't successful because an error was encountered"
This was because the target machine you have elected to be your File Share Witness must have the Exchange Trusted Subsystem group in it's local Administrators group, simply add it to the group and the DAG will successfully create.
This off course creates a problem if you are trying to install the File Share Witness on a Domain Controller, as DC's do not have any local users or group you simply cannot add the Exchange Trusted Subsystem group into the local Administrators. Therefore meaning the File Share Witness cannot be hosted on a DC.