Beim Öffnen meiner Mind Map wird nur eine leere Fläche angezeigt

This is most likely caused by a mismatch in version between the two copies of OpenMind you used to create and edit the document. Both copies must have the same version number; to check this you can use the help -> about menu item and take note of the build number. Higher build numbers of OpenMind can always open mind maps created in lower build numbers of OpenMind but the opposite may not be possible.

If the versions do not match you should immediately update to the latest service pack, on both copies if both build numbers are lower than the latest release.