When a user clicks the Save / NetDocs button in the Home ribbon as illustrated below and Outlook freezes, it could indicate that there are too many temp email files in the Windows temp folder.
This will occur if a user selects emails to upload into ND and does not follow through with the action. ND will not delete the temp files from the local machine.
If you go into the temp folder and remove all files called "message.msg", the Save / NetDocs functionality will return to normal.
Windows default temp location: