Thumbnail Generation Alert! GD Library installed

Hello,

I get this message:

Thumbnail Generation Alert!
There is a problem generating thumbnails for your gallery. Please check that your hosting provider has the GD Image Library extension installed and enabled.
If thumbnails cannot be generated, then full-sized, uncropped images will be used instead. This will result in slow page load times, and thumbnails that do not look correct.

and I checked previous support tickets with same issue.

Under System Info, I get this:

FooGallery version : 1.9.8
WordPress version : 5.3.2
Activated Theme : GeneratePress
PHP version : 7.2.28
PHP GD : Loaded (V2)
PHP Open SSL : Loaded
PHP HTTP Wrapper : Found
PHP HTTPS Wrapper : Found
HTTPS Mismatch : There is a protocol mismatch between your site URL and the actual URL!
PHP Config[allow_url_fopen] : 1
PHP Config[allow_url_include] : 1
Thumbnail Generation Test : https://mysite.com/wp-content/uploads/2020/03/logo_3-small.png
Extensions Endpoint : https://raw.githubusercontent.com/fooplugins/foogallery-extensions/master/extensions.json?v=gaZH!#wFNgV#
Extensions Errors : Nope, all good

I can see that PHP GD is loaded but don’t well understand what is the HTTPS Mismatch as I get no other errors elsewhere, the site is loading correctly and there are no other SSL warnings about mixed content.

I see also that the Thumbnail Generation Test is loading an image from a HTTP url instead of HTTPS (path is edited). Also, this file mentioned here for the test is not part of any gallery.

I get the same with Thumbnail Generation Test:

Thumbnail generation test failed!object(WP_Error)#2551 (2) { [“errors”]=> array(1) { [“invalid_image”]=> array(1) { [0]=> string(21) “File is not an image.” } } [“error_data”]=> array(1) { [“invalid_image”]=> string(76) “https://mysite.com/wp-content/uploads/2020/03/logo_3-small.png” } } array(2) { [“ext”]=> string(3) “png” [“type”]=> string(9) “image/png” }

as it simply takes the latest image in Media Library, even if this image is not part of any gallery, with HTTP instead of HTTPS.

and checking Override Thumb Test doesn’t change anything.

How to correct this?