Steps to reproduce:

(using Moodle QA testing site)

1. Log in as a teacher in a course and create an assignment, set Submission types to 'online text', tick Feedback comments and set 'Comment inline' to 'Yes'.
2. Log in as a student and navigate to the assignment. Click Add submission; type 'This is my class work' and save the changes.
3. Log back in as the teacher; navigate to the assignment>View all submissions and click Grade to access the student's text.

Error will appear:

servicenotavailable
Web service is not available (it doesn't exist or might be disabled)

File: .../lib/externallib.php
Line: 205
Stack trace:
Error code: servicenotavailable

Browsers tested

Latest version of Chrome for Mac and Edge on Windows, both show same error indicating this is not browser or OS related