Config will not save

I host my own sites on an Ubuntu 16.04 server.
I have 3 websites with the latest version of Mautic running on each.

All worked until I needed to change the IP address on the server.
Now 2 of the Mautic installs still work but the third has an issue.

When I enter Configuration and then click either apply or Save and exit, I get Please Wait…
Nothing happens until I click the back button on the browser.
Changes are not saved.

On the working Mautics, when I exit and save a box appears at the bottom of the browser that displays “waiting for www.xxxxxx.com

On the non-working Mautic, that box does not appear.

Any idea how to fix this?

Thanks