Norman,
the only real solution for this is to send an email to help@geni.com
Shmuel-Aharon Kam,
Geni Curator
It is probably a problem on your own PC.
Try this mini tree with father, mother and child:
http://www.geni.com/family-tree/index/6000000010609106763
If it displays correctly you should go into the Preferences in the tree view and reduce number of ancestor and descendants generations to display before you try to view another tree.
In any case: A lot of people with old versions of Internet Explorer have reported similar errors, so I recommend that you try another browser, for example Google Chrome.