In case anyone else encounters this - I also had this problem due to BigBlueButton.
Sidd reported it on the plugins page and and there is a patch which will be in the next BigBlueButton release. In the meantime the patch is here: https://github.com/blindsidenetworks/moodle-mod_bigbluebuttonbn/commit/a5da03712b2e5f9beaa5fddcd26c82b2a61fec48
I have applied it and it works