Customise error: Looks like something’s gone wrong …

Hi, When I try to publish changes to Customise e.g. Fonts, Colours I get the following error:

Looks like something’s gone wrong. Wait a couple of seconds, and then try again.

When I retry same error message recurs and changes don’t take effect.

This is a brand new installation of WordPress 5.8.2 and happens when using themes that support Full Site Editing e.g. Blockbase, Zoologist and the Gutenberg plugin.

If I use themes that don’t support Full Site Editing e.g. Twenty Twenty-One changes to Customise publish fine.

Any ideas appreciated?