Hello Frank,
Do you use another host header to access the SharePoint site? E.g. http://portal. If so, please try that to see if it resolves the issue.
If not, please can you enable Debug logging by clicking on the question mark icon on the factbox | About | Change log level to Debug.
Restart NAV client and reproduce the problem by dragging and dropping a file.
Navigate to the user's TEMP folder and get a copy of the 'Zetadocs' subfolder which should include the file 'ZetadocsCaptureAddin.log'.
Send the files to support@equisys.com
Kind regards,