We do our best to keep Mailster work best with most common plugin by following the WordPress Coding Standards but sometimes Mailster or a third party plugin breaks caused by the other.
Best is to disable one of the plugins and check if the other is still not working. If so the plugin you’ve disabled isn’t causing the problem and you should disable additional plugins to check if any other plugin is causing conflicts.
You should also test if this error happens when the default WordPress theme is active. Otherwise your current theme may causes problems.