The problem is coming from the simple error handler. The Simple Error Handler cannot decide if plots error + warnings, or only errors.
You cannot put your enumerator to the simple error handler, it works only with the general error handler. In your program you are giving to the simple error handler a number 3 as error code. The VI convert this error number 3 with the error GPIB you received.
You should so replace the Simple Error Handler.vi with the General Error Handler.vi
If I pass error code 3 to either the simple error handler or the general error handler, the exact same message is generated. As the message shows, error 3 can have 2 possible reasons. Since there is no GPIB involved, there is no reason to think that one of the possible reasons is the one about the controller not addressed correctly.