Featured Image not working (f8 theme)

Hello,

I am using the f8 theme, have done for several years.
I have a static header image for the site but when you click on posts the header changes to the featured Image of your choice in your post.

a few days ago, this stopped working. No idea why
Can anyone help or does anyone have idea’s on how to fix this mysterious problem?

https://henryliamcollins.co.uk/blindfoldeddrawings/

Many Thanks,

Henry