In the jenkins setup wizard, on the "Getting Started \ Customise Jenkins" screen where it asks you to either "Install suggested plugins" or "Select plugins to install", if jenkins is running slowly, eg an NFS filesystem, slow machine, or you are installing remotely over a slow connection, clicking either button gives an error after 10 seconds

An error occured

Unable to connect to Jenkins

The reason for this is simple: in static/c9073345/jsbundles/pluginSetupWizard.js, there's a timeout (line 1919 in 2.60.3):

// default 10 seconds for AJAX responses to return before triggering an error condition
var pluginManagerErrorTimeoutMillis = 10 * 1000;

10 seconds isn't long enough on this setup.  Changing this to 180s solves the problem (it actually takes roughly 30 seconds for me)

// default 10 seconds for AJAX responses to return before triggering an error condition
var pluginManagerErrorTimeoutMillis = 180 * 1000;

It's hard to see a benefit to a low timeout here : the error isn't handled in any way and there's no workaround or other way to get past it except to tweaks this code in the browser's javascript debugger. 

FYI, on my setup, the page /pluginManager/plugins that is timing out takes 18s to actually  load once I increase the timeout although I imagine it could be higher than this for others.