Synchronize your GMX Organizer with your calendar application on Thunderbird to make sure, that your appointments are up to date.
If you use the "Lightning" add-on for Thunderbird, you can configure automatic synchronization. Due to the technical limitations of Thunderbird, you have to synchronize each of your calendars (default calendar and other calendars) individually.
Example: For the e-mail address john.doe@gmx.com, you enter https://calendar.gmx.com/begenda/dav/john.doe@gmx.com/calendar/ as the location.
If you want to synchronize other GMX Organizers with Thunderbird, repeat steps 4 through 14.