One user had this error and he eventually solved it by refreshing the design of bookmarks.nsf .
The designer has it's own bookmark list but they live in the bookmark.nsf database.
NB: Make a back up copy of bookmark.nsf before you do this, just in case.
File ->Database-> Refresh Design, then click on local and let it complete.
Check out http://www-10.lotus.com/ldd/nd6forum.nsf/55c38d716d632d9b8525689b005ba1c0/a8ecc6c87aedc44a8525712400517dda?OpenDocument&Highlight=0,entry,index,client which I found on the subject.
It explains that dragging a bookmark from the client to the designer made a mess of his bookmark.nsf file and advises not to do that!