Incomplete enrollment notification-Getting server error after sending mail with attachment

Environment: Codemap
Company: For QA KinderCare
Page: Incomplete Enrollment Notification

Steps:
1.Login as partner
2.Go to Incomplete Enrollment Notification
3.Select employee
4.Add attachement
5.Enter message
6.Click on send button

Actual result: Getting server error
Expected result: Email notification send successfully pop up should be display.

Note-Same issue exist on Old UI. Verified on stage for 'Austin for hspl' Company.
Please refer attached screen shot and error log