Unable to save when choose a particular resource in internal service request

Summary:

We have problem to save for one particular assigned to person.

This user have resource role and help desk agent role. We can select the name in the drop down list. But it give the error when we save the record.

Content (please ensure you mask any confidential information):

NA

Version (include the version you are using, if applicable):

25C

Code Snippet (add any code snippets that support your topic, if applicable):