Quoted from @Lynne for sake of clarity:
If you think the problem is a plugin, then disable your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit includes/config.php and add this line right under <?php
Code:
define('DISABLE_HOOKS', true);
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You can also do this in your admincp.
AdminCP -> Settings -> Options -> Plugin/Hook System -> Enable Plugin/Hook System
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