This error was fixed in v7.6.0; basically the memory/heap table gets full and isn't purged unless a new post is made; sites with little activity will see the tables getting full.

The initial solution was to purge and recreate the table (see this post from 2007).

The fix was to add the trigger to the login page, which results in the cache purge occurring every time the login page is accessed (versus just when a post is made):
Open /scripts/login.inc.php and Find:
Code
trigger_subscriptions();
Add Below:
Code
 // TRIGGER: Update the views
    update_views_counter();

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