I get this notice (Looks like something’s gone wrong. Wait a couple seconds, and then try again.) every time I try to publish changes under Theme – Customize, trying to add header image, add slide or change menu, etc. Please help.
The issue you mentioned may have been caused due to the plugins used. So, can you try disabling the plugins used one by one and check? If the problem still persists, can you provide us URL to your site so that we can inspect it and give you a proper solution?
Do you get the same issue with default theme like Twenty Seventeen? Can you try to check by switching theme to default? If the problem still persists in default theme also then it is probably due to the plugins used. If not, do let us know.