First thing I would check is: $wgMaxImageArea = ' some value ';
See if it is accidentally explicitly set to a smaller value in LocalSettings as this will
make
thumbnailing fail. e.g. 400000 for 2000px X 2000px image would prevent thumbs on a
2200x1900
image which is 418000 in size or any combination widthxheight bigger than the max.
Take care
Tom