possible, that you add some pic's via FTP there? maybe, a hidden file inside of this folder (not every ftp-client show all files in the default settings).
i test it with different PHP-Version (7.2.0, 7.2.8, 7.3.0), no problems
this part of the script build's the preview-thumbs for this section
the 4 warnings in your last message show's everywhere the same problem. this four functions works with a path or the informations from there. (for example: if getimagesize() has no informations about the size, this size is missing in the next function imagecopyresampled() ). equal in all this functions is the source path to the image. Ruud write the answer: a image (or the info inside of the image) is broken or the selected file is not an image
try to call all images in this folder step by step in a browser. you can also delete all the thumbs in this yolder /media/minigal2/[your-section-id]/thumbs and reload the section in you wb-backend again, then compare the pictures in this picture folder with the pictures in the thumb-folder. normalize, the script breaks in this step with the result, that one pic is missing in the thumb-folder - so you found the source picture with the problem
if the problem coms again, we can set some exeption inside of this function, so that you see, what the source is