The issue was identified because forum posts were not being emailed. It has temporarily been resolved by disabling \core\task\legacy_plugin_cron_task - but what effect will this have on Quizzes or any other mods that may be using legacy cron?
To be honest, I don't remember exactly what the situation was in Moodle 3.5.
There was a big push to get all the legacy cron converted to use the new scheduled task system (MDL-61164). Some of that was done for 3.5, but some of it was not finished until 3.7 (See the linked issues.)
There was a big push to get all the legacy cron converted to use the new scheduled task system (MDL-61164). Some of that was done for 3.5, but some of it was not finished until 3.7 (See the linked issues.)