I use Marble 2.2.20 from KDE Applications 17.08.1. https://phabricator.kde.org/D5650 introduced a new feature that allows to rotate the globe around the third (previously fixed) axis using right click and drag. If you accidentally trigger this feature (which is very easy, see bug #389578), there is no easy way to return the view to normal. Even returning to the home location will still keep the globe rotated. And there is also no snapping to the 0° angle.
The one reliable way to undo the rotation appears to be to close Marble and reopen it.
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This bug has been in NEEDSINFO status with no change for at least 30 days. The bug is now closed as RESOLVED > WORKSFORME due to lack of needed information. For more information about our bug triaging procedures please read the wiki located here: https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging Thank you for helping us make KDE software even better for everyone!
This was only "fixed" in so far as right-click rotation can now thankfully be disabled altogether. But if it is enabled, there is still no easy way to return to the default orientation.