sshs1995 I somehow cannot see thumbnails. Can somebody please help me with this?
Do you have a link? What does your ?check=1
page say? See troubleshooting. What do you see if you open browser developer tools > network [images] and reload the page, then click on an image error and "open in new tab"? You will see the error with description.
This to me looks like PHP GD is not installed correctly. It looks like it's available (else, Files would not attempt to load resized images), but the GD is outdated or the internal libraries used are. Alternatively, you are missing PHP mbstring extension, and then your images are failing when attempting to read image EXIF data. See this post:
https://forum.files.gallery/d/41-error-with-jpg-files