Unable to save form template/Error Saving Form template

Summary

When saving a form where the template contains the Email field, it will fail with:

Error: Saving template form failed. A server error occurred while trying to save the template form.

Steps to reproduce

1. Go to Project settings > Forms > Create form.
2. Select a template that contains the Email field, for example, ​New Employee Onboarding Request > Insert template.
3. Make the desired modifications and click Save changes.

Screen recording: Screen Recording 2022-01-19 at 12.33.27.mov

Expected Results

The form is saved normally.

Actual Results

It shows Error: Saving template form failed. A server error occurred while trying to save the template form.

Developer tools:

{"error":"json","details":[["/design/questions/17/validation/rgx/p","JsonValidationError(List(error.expected.jsstring),WrappedArray())"]]}

Workaround

Remove the Email field and add it again.