The dialog box gives an error as shown below.
The site is all the same.
Please help if you know the cause.
Ubuntu 18.04 version, Apache 2.4, php7.2 version and I am using nginx proxy. The cache is using the pagespeed module.
demo site
my site
It most likely isn't in the core as you show it works on the demo. If it were an issue with the SEPHP script, it normally wouldn't work on the demo. As third party products can and do modify source, disabling the plugin you have would not have removed the modifications. You said you are using a pagespeed module. You'll need to check with that expert to see if there is a conflict. If not, then you need to troubleshoot by checking your error logs and perhaps in the browser inspect element but the first place to check is via the expert who made your plugin since you can see it works at the demo.