When we're editing a notification scheme, Jira allows us to select portal-only customers in the single user picker, and when we try to save, an error 500 is displayed:

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If you create a request, include the following so we can help you as fast as possible:

• Error type: 500 - Internal server error
• Log reference: •••••••••

Suggestion

• Implement a specific error message that clearly explains the issue. For example:
  • "Error: Portal-only customers can't be added to internal notification schemes. Please select a different user."

Or

• Modify the user picker to exclude portal-only customers.