I have the same error in MOSS 3.0 with Outlook 2007 when making changes to
the Sharepoint calendar, upon syncing with the Sharepoint list, the changes
are not uploaded with the identical error specified previously in this
thread. Running on XPsp2 client in VMware session for testing. Are there
host header settings in IIS that must be appended to your default or
Sharepoint site? Using SSL certs with an FQDN for the host header on
Sharepoint site....default is stopped.