Originally Posted by KW802
Does it happen only if you upload a file or does it also happen if you use the 'upload from FTP' option as well?
To rule out that your GD or ImageMagick is configured incorrectly, try uploading a really small image file (like a smiley or something else that is smaller then the dimensions of the thumbnail you have specified) to see if that file is accepted.
Thanks for the reply! It happens with both ways. Sorry I didn't mention this before.
I also tried a very small image with the same result. (favicon.gif)