check plugins. I know its a pain in the a$$, but i had a popup builder that caused the problems. I deactivated the plugin, designed my header, then reactivated and the header worked fine. Each time you want to edit the header you had to do this again. Hope this helps, just my 2 cents
Please deactivate all the plugins not related to the theme and switch the parent theme, check the issue, if the problem remains, leave the plugins not activated and provide the site admin access to the private area