When encountering errors saving and other errors where you get a vague/minimal error in the error message on the network trace, the handler may return an error message that says {System.Web.Exception} without much useful detail
In order to 'dial up' logging, it is necessary to add some diagnostic listeners to your web.config
In this example we're going go assume your login username is YOUR_USERNAME. Obviously, here you will change this to your actual username
Alternately, if you want to set the folder to something more generic, just make sure NOT to use a protected directory such as c:\Temp etc.
Save this out then rebuild and refresh your browser to start logging. You can attach the log to a support case as a text file attachment (PLEASE DO NOT paste long logs or blocks of code into cases directly - please attach as a file attachment)