Some actions in Ministry Sync prompt an alert in your browser, which sometimes require a response. For instance: moving someone to another group, promoting or demoting a host, creating a table chart, cancelling registrations or possibly making a donation.
Checking the box "Prevent this page from creating additional dialogs" will turn those alerts off, and your actions won't be completed.
To turn these alerts back on and continue working in Ministry Sync, completely close your browser and reopen it.