Good morning Thomas,
if I remember my own tries and the information of the respective DocuWare webinars correctly, both DocuWare versions have to be based on the same DocuWare version.
Didn't try it that often, thus I'm not sure, if this means the current version of both DocuWare systems, or if it means the first version of the DocuWare systems, which they had during the installation/creation.
If your source system has now a higher version as it has on your last export, maybe you try to export the dwbuc file again from your source system, and try to import it again into your target system, if both have (now) the same version.
Best regards,
Frank