How to deleting unuse image sizes?

I want to delete unused image sizes. So within WPO I go to “images” and here I open “Unused images and sizes”.

WPO now is loading data, which takes ages (8 to 10 minutes) – but to be fair, my uploads folder has 7000+ image-files.

However, after the loading WPO gives me two choices: “scan website for unused images” and / or “scan website for unused image sizes”.

I go for the image sizes and receive a result showing me a list with “registered image sizes” and a list with “unused image sizes”. The unused sizes is want I want to delete. So I mark them and at the bottom of the page I click on “Remove selected sizes”.

Now WPO seems to be doing something but returns after a few seconds a pop-up box telling me “0 unused images removed with a total size of 0 B”.

I tried this several times selecting only 1 image sizes as well as 5 and all.

So my question is, how can I delete unused image sizes? This function was one of my main reasons why I paid for the Premium version.

Thanks in advance for your effort to help.

Henning

The page I need help with: [log in to see the link]