Check 'Replace a process level token' account privilege of user which runs Solarwinds Platform Module Engine service

The SolarWinds Platform fails to validate the login information required to run a test on a customized alert or action, for example, Configure Action: Execute an external program. The following error is displayed after entering the login credentials:

Check ‘Replace a process level token’ account privilege of user which runs SolarWinds Orion Module Engine service.

1. Create a SolarWinds user account with administrator privileges (local or domain).
2. Open services.msc and configure the SolarWinds Platform Module Engine service to log on as the user account previously created.
3. Add full control permissions to the script folder for the user account.
4. Add the user account to the Replace a process level token local security policy (refer to Microsoft resources for details on this task).
5. Restart the SolarWinds Platform services.

You can also try using the User Principal Name (UPN) login convention, such as, user@domain instead of domain\username on the test page of the alert or action.
 

If all else fails see this article: Cannot execute the external program or visual basic script alert actions