"Not_Set" is an invalid hostname. Please enter a valid hostname

Error Message

When attempting to launch ArcMap, the following error message is returned:

Error:   
"Not_Set" is an invalid hostname. Please enter a valid hostname.

Cause

The error occurs when a 'Concurrent Use' license level is selected in ArcGIS Administrator, and no License Manager is defined.

Solution or Workaround

To circumvent the issue, follow the steps below:

1. Launch ArcGIS Administrator.
2. Ensure the correct license level is selected.
3. If a Concurrent Use license level is selected, click 'Change...' to define a License Manager.

4. Enter the name of the License Manager in the available field. Alternatively, click 'Browse...' to connect to a specific computer on the network. Click OK.

5. Click Apply, and OK.
6. Close ArcGIS Administrator, and relaunch ArcMap.