Steps to replicate:

1. Access a test course and add the 'Activities' block
2. Create a new forum activity in any topic section (not the general section)
   1. Enter a title
   2. Set Forum type to 'A simple single discussion'
   3. Set 'Whole forum grading' to Type>Point
   4. Save and return to course
3. Click 'Forums' in the Activities block
4. Click the link to the forum to be taken to the forum page
5. Click 'Grade users'

Expected result:

User grading screen appears

Observed result:

No grading screen appears. The following error appears:

invalidrecordunknown

Can't find data record in database.
File: /lib/dml/moodle_database.php
Line: 1627
Stack trace: SELECT c.* FROM {course_modules} cm JOIN {course} c ON c.id = cm.course WHERE cm.id = ? [array ( 0 => 0, )] Error code: invalidrecordunknown * line 1627 of /lib/dml/moodle_database.php: dml_missing_record_exception thrown * line 2237 of /lib/modinfolib.php: call to moodle_database->get_record_sql() * line 4089 of /course/externallib.php: call to get_course_and_cm_from_cmid() * line ? of unknownfile: call to core_course_external::get_enrolled_users_by_cmid() * line 261 of /lib/externallib.php: call to call_user_func_array() * line 81 of /lib/ajax/service.php: call to external_api::call_external_function()
 
Other notes:
**I noticed that this only occurs when accessing the forum link via the Activities block, which gives a link that looks like _https://sandbox.moodledemo.net/mod/forum/view.php?f=3_
 
However, when accessing via the course page, it doesn't occur and instead the link looks like: https://sandbox.moodledemo.net/mod/forum/view.php?id=4
 
Tested and can replicate on https://sandbox.moodledemo.net