SharePoint 2013: Sorry, we’re having trouble reaching the server.

Problem

You see this message appear when entering usernames, whether for adding new users, using the Check Permissions tool, etc:

Another thing that occurs is that no results drop down appears, listing possibilities as you type.  On the other hand, if you enter the full username, and then click Check Now or Share, the username is in fact accepted and things subsequently happen the way you expect them to happen: permissions are checked or the new username is added to the SharePoint group you wanted to add it to.  You also notice something else.

Completely unrelated to this (or so it seems) is another error message that appears when users attempt to modify list item or document properties.  When users make changes to these properties, and then click OK, the property form remains visible for what seems longer than it should, and then a message is displayed on this form:

and

Troubleshooting

1. Check server event logs:
• No events occurring that occurred at same time as the message
2. Check ULS logs:
• You see an interesting series of error messages that recur:

  Unknown SPRequest error occurred. More information: 0x80070002
  
  System.IO.FileNotFoundException: The system cannot find the 
  file specified. (Exception from HRESULT: 0x80070002), StackTrace...
  
  Access Denied for /_vti_bin/client.svc/ProcessQuery. StackTrace...
  
  Redirect to AccessDenied.aspx?Source=
  https.../_vti_bin/client.svc/ProcessQuery failed. Exception: 
  System.Web.HttpException (0x80004005): Cannot redirect after
  HTTP headers have been sent. 
   
  

3. Test other site collections:  
• Problem still occurs.
4. Test other web applications: 
• You find that this problem occurs for only one of the web applications in your farm and not all.  For example, it does not occur for the Central Administration web application.
5. Create a new web application and site collection and import problematic web application content: 
• You find that the problem does not occur.
6. Extend the problematic web application:
• You find that the problem does not occur
7. Unextend the problematic web application:
• You find that the problem still does not occur.

Solution

• Extend and then unextend the problematic web application (see reference 7).

References

1. SharePoint 2013 People Picker error: “Sorry, we’re having trouble reaching the server.”
2. SharePoint 2013: The server was unable to save the form at this time. Please try again. [for lists]
3. SharePoint 2013: The server was unable to save the form at this time. Please try again. [for document libraries]
4. SharePoint 2013 People Picker error: “Sorry, we’re having trouble reaching the server"
5. SharePoint 2013: People Picker Error
6. The server was unable to save the form at this time. Please try again
7. Error while creating task: The server was unable to save the form at this time. Please try again. (SharePoint 2013)

Notes

• There are temporary fixes that can be easily implemented with respect to the list item or document property issues, and these fixes involve adding content types (see references 2 and 3).  However, these fixes do not resolve the PeoplePicker error message.
• The web application extension used for this solution was a simple one using all defaults: a host header (DNS) was not used for the URL - just the web server name.
• I worked with Microsoft Support to resolve this issue and the apparently related list item and document property issues (see references 2 & 3).  The solution suggested by the support technician was to extend the web application, and this was the solution that ultimately fixed all of the issues noted in this posting.  However, why this one farm web application began presenting this behavior and not the others is still unknown.