by Pete Jones.
I'm using Moodle 2.6.2 and was getting the same error message when I submitted an online text assignment as a student.
I've since worked out how you can avoid the problem by changing some settings.
In Site administration - Plugins - Activity modules - Assignment - Assignment settings, turn off 'Send submissions receipt to students'.
And in the settings for the assignment itself, set 'Notify graders about submissions' to 'No'.