Sorry for the delay in replying – I’m 99% sure the problem here is caused by me. The website is a test website which I haven’t used for some time. It was locked down from external access (which I’ve removed temporarily), and also the SSL certificate is currently out of date which I think may be the cause – I’m in the process of getting this updated.
Running a web inspector, the javascript console is showing this when I hit the “Update” button:
POST https://sitenamehidden/wp-admin/admin-ajax.php 403 (Forbidden)
send @ load-scripts.php?c=0&load%5Bchunk_0%5D=jquery-core,jquery-migrate,utils,wp-hooks&ver=6.6:2
ajax @ load-scripts.php?c=0&load%5Bchunk_0%5D=jquery-core,jquery-migrate,utils,wp-hooks&ver=6.6:2
(anonymous) @ load-scripts.php?c=0&load%5Bchunk_0%5D=jquery-core,jquery-migrate,utils,wp-hooks&ver=6.6:5
e.<computed> @ load-scripts.php?c=0&load%5Bchunk_0%5D=jquery-core,jquery-migrate,utils,wp-hooks&ver=6.6:5
ur_save_form @ form-builder.min.js?ver=3.2.1.3:1
(anonymous) @ form-builder.min.js?ver=3.2.1.3:1
dispatch @ load-scripts.php?c=0&load%5Bchunk_0%5D=jquery-core,jquery-migrate,utils,wp-hooks&ver=6.6:2
v.handle @ load-scripts.php?c=0&load%5Bchunk_0%5D=jquery-core,jquery-migrate,utils,wp-hooks&ver=6.6:2
So clearly some website configuration issue is causing the 403 error.