Thread:Project:Support desk/Upgraded old MW site to 1.23.3 and thumbnails stopped working/reply

Well, in the debug log you should see the paths of the files it's trying to generate.

I'd suggest you to try loading thumb.php on your wiki to generate a thumbnail (specifying the correct parameters) and see if it throws the same error. Then go to the debug log and look at what's trying to do.

Pay special attention to all file paths it's accessing, to see if any file path is missing, or the directory structure it expects is different. Check also file permission issues.

Also look if in the debug log it attempts to run imagemagick, and if the error comes after the call to it. If that's what happens, try running the command directly (the debug log prints the command as it's being executed by MediaWiki). You may need to increase $wgMaxShellFileSize, $wgMaxShellMemory and Manual:$wgMaxShellTime.