Just ran into this running on BlueHost. Thank you @LeonidS ! This was my issue. Sat on this as it wasn't a priority, but getting this resolved on our Hosting Provide with your recommendation of:

UPDATE for the similar cases: In some servers, the possibility to create several threads can be closed, so then it needs to add the following line to the inc/header.inc.php file:

define('BX_SYSTEM_FFMPEG_THREAD', 1);

This was huge!