We do our best to keep Mailster working best with the most common plugins by following the WordPress Coding Standards but sometimes Mailster or a third-party plugin breaks caused by the other.

The 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 cause problems.