This error is often due to some OpenDoc parts being missing or damaged.
Open the System Folder, then open the Editors folder, then open the "AppleShare IP Admin Libraries" folder. Inside that folder should be two items, "AppleShare IP Admin Editors" and "AppleShare IP Admin Lib". If one or both of these files are missing, then run the AppleShare IP Installer and then verify that they are there.
Figure 1, Contents of "AppleShare IP Admin Libraries" folder.
If it persists, stop the AppleShare IP Servers, pull the OpenDoc prefs from the Preferences folder, restart and try the Web & File Admin again. If it still will not open, remove and reinstall OpenDoc.