Unfortunately, this plugin (at least version 5.2) breaks core functionality of WordPress 2.x by breaking the Pluggable functions architecture.
Line 37 of the plugin forces pluggable.php to load. This is *very* undesired.
WordPress is architected to load pluggable.php after all plugins load. This is a base assertion. Because this plugin forces pluggable.php to load out of order, it causes ALL PLUGINS that load after this maintenance mode plugin to NOT BE ABLE TO PLUG FUNCTIONS.
See how the pluggable functions work:
Then see wp-settings lines 595-601 to see how plugins and pluggable.php are loaded.
No known workaround.