How to troubleshoot spacenavigator NDOF in Blender and Linux?

Hi,

I have used this space navigator (by 3dconnexion) for a long time in Windows and Blender and started to really appreciate it. Now I’m trying to get it working under Linux (Debian Stretch), and am running into problems. The driver from the manufacturers website does not work anymore, but there is an opensource replacement http://spacenav.sourceforge.net/, which is supposed to work.

I installed it (included in Debian) and started the service and got this:
spacenavd[815]: failed to open device: No such file or directory
california spacenavd[815]: already using device: 3Dconnexion SpaceNavigator (/dev/input/event4)
california spacenavd[815]: failed to find any supported devices
california spacenavd[815]: failed to open device: No such file or directory
california spacenavd[815]: already using device: 3Dconnexion SpaceNavigator (/dev/input/event4)
california spacenavd[815]: failed to find any supported devices
california spacenavd[815]: failed to open device: No such file or directory
california spacenavd[815]: already using device: 3Dconnexion SpaceNavigator (/dev/input/event4)
california spacenavd[815]: failed to find any supported devices

When I start Blender (via Steam) in Linux, it reports:
ndof: unknown Logitech product c01e
ndof: using SpaceNavigator

So it seems to be working, but it is not, I can’t move the viewport or anything else.
How can I troubleshoot this, what seems to be the problem that Blender cannot use it?