Error when publishing from Appearance Customize

I am getting this error when I try to publish changes to my main page via the APPEARANCE > CUSTOMIZE page editor in Colibri WP Pro. The theme is MagnusWP. Everything has been going great until I tried to use the HUSTLE plugin last night, and now I get this error. I have removed the plugin completely, and the page finally did publish once, but now it won’t let me publish other changes, although the page does save.

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

I have disabled all plugins, changed themes (which worked, so I think it’s a theme issue?) and even tried making different changes on the page, but always this error.

I have maintenance mode on so there is no way to show the link. It looks ok anyway. This is something internal inside ColibriWP editor and/or WP itself.

Thanks!