Add new plugin search not working

When I try to search for a new plugin to add (Plugins > Add new > Search) I keep getting the following message:

An unexpected error occurred. Something may be wrong with WordPress.org or this server’s configuration. If you continue to have problems, please try the support forums.

I can’t find a solution.

It doesn’t happen all the time, but it happens most of the time.It happened on WP4.7 and upgrading to WP4.8 hasn’t fixed it.
It happens on IE, Firefox and Safari on both OSX and Windows.

Any thoughts?