This error is triggered when you request the upgrade of an installed plugin from manage_plugin_page.php. Without detailed information about what that plugin's schema() method is trying to do, there is not much I can do to help, but you can have a look there.

You may also want to configure your system to display detailed errors,:

1. temporarily configure your system as follows:

   $g_display_errors = array(
   E_WARNING => 'halt',
   E_NOTICE => 'halt',
   E_USER_ERROR => 'halt',
   E_USER_WARNING => 'halt',
   E_USER_NOTICE => 'halt'
   );
   $g_show_detailed_errors = ON;

WARNING - SECURITY RISK: the 'show_detailed_errors' config can cause MantisBT to display sensitive information about your system. We recommend to restrict its activation to a Test environment, only for as long as necessary. If possible, do not turn it ON globally, instead limit it for specific user(s) using the Manage Configuration page.

This is not a bug or feature request for MantisBT (you are asking for help on how to debug a plugin's upgrade process). I am therefore resolving this issue as "no change required".

Please use the forums to get support on customizing and using MantisBT (refer to http://www.mantisbt.org/support.php for links and further details).