A user's Windows user name is changed in Active Directory, and using either DocuWare User Synchronization or a manual update of DocuWare the Network Id entry is changed to reflect the new Windows name.
When trying to log in to DocuWare using Single Sign-on the old Windows User name is used, and the log in fails.

This is not a problem with DocuWare. The problem is that the users Locate Security Authority cache has not been up dated.
Please see this FAQ from Microsoft to resolve the problem:
Note! The cache on the Web Server and DocuWare Server (if seperate to the Web server) may also need to be cleared.
After clearing we recommend a restart of IIS and the DocuWare Services.