Hi @Andres.Aulasneo @Anh_Vu_Nguy_n
Certainly there is something related with lms-worker / cms-worker connection. When it get stucked, neither lms-worker or cms-worker logs anything.
We were also having problems with other celery tasks as in this post Celery tasks are broken - Site Operators / Site Operations Help - Open edX discussions, so after cleaning up Redis caché the instructors tasks worked again and the issues with emails like password reset and user validation were fixed.
But we still have a problem with course inscription bulk emails. I have to check @Anh_Vu_Nguy_n proposal, I will post again afterwards
Thanks very much for your help