Moodle: 3.7.1+ (Build: 20190712)
Steps to reproduce:
1. Create a test course (any size will do)
2. Make sure that course is visible.
3. Make sure at least one teacher is enrolled.
4. Make sure at least one student has suspended enrollment.
5. Login as the teacher.
6. Go to the participants list for the test course.
7. Select some students in the participants list making sure that one of them is a suspended enrollment.
8. Select "Send a message" in the dropdown at the bottom of the list.
9. Attempt to send a message to the selected students.
After sending the message the modal disappears and a success message displays.
An error dialog appears with the heading "formatnotsupported" and message "webservice/formatnotsupported"
The messages all appear to get sent properly.