Thread:Project:Support desk/Image thumbnails broken

The image thumbnail function on my entire wiki is broken... it can only show images at full size, or previously-created thumbnails. It can't seem to create any new thumbnails of any size.

Any thumbnail only has a grey box that looks like this:

The only options related to ImageMagick in my LocalSettings.php file are:
 * $wgUseImageMagick = true;
 * $wgImageMagickConvertCommand ='C:\mowes\ImageMagick\convert.exe';

(Hiding the $wgImageMagickConvertCommand option changed the error to "Error creating thumbnail: The system cannot find the path specified.")

All other options (relating to SVGs, which never worked: see here) were removed to try and undo this glitch, but it didn't fix anything.

Please help. This problem has crippled my wiki since early August.