SharePoint 2013: The server was unable to save the form at this time. Please try again. [for document libraries]

Problem

Your users report seeing the following error message, when attempting to save changes to document properties:

This is an externally-facing SharePoint 2013 instance.  You are able to repeat the user experience, and you find that it occurs regardless of browser type.  The Free Download Manager is not deployed to your users, and restarting the SharePoint Search Host Controller, on the SharePoint application server, has no affect.

NOTE: this posting engages this problem for Document Libraries.  Not for lists.  This error message can also appear when attempting to modify list item properties.   If you see this for lists, please see this article.

Solution

1. Go to your  document library's Settings page.
2. Scroll down to the Content Types group.  By default, you'll see just one content type here:
   
   .
3. Now scroll back up, and then click Advanced settings.
4. On the Advanced Settings page, look for the Content Types group.
   
   .
5. Select Yes, and then click OK.
6. Back on the Settings page, scroll down to the Content Types group, and then click Add from existing site content types.
7. In the Available Site Content Types multi-select box, select Link to a Document.
   
8. Click Add >, and then click OK.
9. Back on the Settings page, again scroll down to the Content Types group, and now you will see two content types there
   
10. The change is implemented immediately.

References

• The server was unable to save the form at this time. Please try again
• SharePoint 2013: The server was unable to save the form at this time. Please try again. [for lists]

Notes

• Thanks to ABitEncrypted for finding this solution.
• This solution works for Document Libraries.  There is no "Link to Document" content type that can be added to lists.