Original issue:

MDL-70801 adds the 'mycourses' layout to the Boost and Classic themes, it also defines in their 'config.php' a region called 'content', however this is not implemented!  Also, Boost defines the language file 'region-content' string, but Classic does not.

Therefore as the 'mycourses' page does not require block regions then 'content' needs removing!  i.e. with an empty 'regions' array in config.php for 'mycourses' just like 'login'.

UPDATE:

Found the line:

echo $OUTPUT->custom_block_region('content');

in '/my/courses.php'.  Strange there is no '$PAGE->blocks->add_region('content');' line as with other files using this method.  So... to avoid confusion with theme developers, then perhaps that is a better solution and not have 'content' listed as a block region in the theme's config.php file.