Thanks for the reply, Ken.
Turns out it was similar to another incident https://moodle.org/mod/forum/discuss.php?d=407618 Guest user did not exist anymore in the mdl_user tbl. I am not sure how this has happened, as other posts on the incident seem to suggest it was after they ran an upgrade but we haven't upgraded or made any changes (as far as I am aware). Not sure how it has been removed. To be brutally honest, not too sure how the 'Guest user' is related to editing, but will look into it to see how how this impacts editing courses. Anyway, re-added Guest user and it is working again!
Turns out it was similar to another incident https://moodle.org/mod/forum/discuss.php?d=407618 Guest user did not exist anymore in the mdl_user tbl. I am not sure how this has happened, as other posts on the incident seem to suggest it was after they ran an upgrade but we haven't upgraded or made any changes (as far as I am aware). Not sure how it has been removed. To be brutally honest, not too sure how the 'Guest user' is related to editing, but will look into it to see how how this impacts editing courses. Anyway, re-added Guest user and it is working again!
Thanks for help.