I know this has been covered on a number of sites, but I had not seen this error before. I have always said that the best way to extend the schema was to use an LDIF file (LDAP Data Interchange Format) I prefer it as 'you are in more control' of the schema extension, and can see what changes...
Microsoft has released the ConfigMgr SDK. You can get the download the Microsoft Download Center It includes DCM Digest Authoring guide, DCM digest schema, a SUM gadget that gives you current status messages on patches you have deployed, samples on creating admin console extensions, OSD custom task sequences...