causes edit and view screens not to load

I've just downloaded and installed addnode (with great excitement -- just what I've been looking for) but when I change a node field to the addnode nodereference widget option, when I go in to edit or create that node, I get a blank (i.e. unloaded) page.

Similarly, if I click on configure to configure the field in Manage fields (e.g. to change it from add node list) I get the same blank screen. Using 'view source' for the blank page reveals nothing -- no page content is loading.

Have I done something wrong or is further configuration required?

Thanks!
Brian

DP:5.7, PHP:5.1.2