Vantage Premium Customizer does not allow editing masthead background image

I run a website with Vantage Premium for several years now and have always lots of extra work when WP auto-updates, as well as plugin updates to follow the upgrade processes.
Currently, I am on WP 4.7.3 and Vantage Premium 2.5.4 . Some time ago, I moved the whole site between server folders. I could make almost everything work properly, only the image used as the background image in the masthead area still points to the old folder (which I want to delete eventually). Now I was implementing https throughout the site and have the mixed security situation since it points to outside the realm of the certificate. In the past, the configuration happens through the theme settings page. Now its ‘upgraded’ to use the customizer. However, the field to disable or change the image I cannot find.
I checked that the file exists in the media library in the new location. It has the correct entry in the post table of the database, but there seems to be no other place where this is stored. Help, please.