By default the iView DedicationAlerter of portal component sap.com.portal.supportability.isolde is called. You can display a different iView (e.g. with your own error message text).
Procedure
Choose System Administration → Support → Support-Desk → Portal Runtime → Workload Distribution Configuration.
Click on folder Workload Distribution Configuration.
Go to edit mode for serviceDistributionConfiguration.
In the fieldDefault Alert iView Name enter the desired iView path and click on Apply.
Note
Do not change the properties HTTP Methods and Use Absolute Path.