I think I saw this error before. I would check, if you are using the lastest version of OrderedComposite from HEAD:
http://fisheye.openacs.org/browse/OpenACS/openacs-4/packages/xotcl-core/tcl/20-Ordered-Composite-procs.tcl
Please let me know if the error still shows up using the very latest version of this file.
Thank you! Updating xotcl-core fixed it!
Thank you Gustaf and everyone for taking the time you did to dwell on this.
cheers,
Torben