[Dark Zero]

Hi hi
in the footer template you deleted this line?

<!-- Do not remove $cronimage or your scheduled tasks will cease to function -->
    $cronimage
<!-- Do not remove $cronimage or your scheduled tasks will cease to function -->

without that code the cron is uncapable of operate.
?what session timeout you use?

also check in the control panel->scheduled tasks for the hourly cleanups and view if are checked and operative (for more check about this also make a view in the task logs for that two)
in a normal operation the cron, exec the hourly cleanup, this delete the sessions with overtime and the active rows are limited to the concurrent users (or bots)
if this fails probably the sessions with overtime are not deleted and the heap table grow up to the memory limit assigned in mysql config.