In some cases Mailster is not able to create certain tables in you database. You may end up finding an error similar to
Table ‘wp_mailster_[tablename]’ doesn’t exists
Visit the update page to force Mailster to create missing database tables by visiting
example.com with your actual domain.
This will run the update progress and problem should be fixed.
You can also deactivate the plugin and activate it again afterwards. This will run a DB check and create missing campaigns.