The snuggletex-upconversion-1.2.2.jar does not work on computers with language settings other than English. I need to start my JVM with the flag -Duser.language=en to get it working.
Cause of this bug is the file: uk\ac\ed\ph\snuggletex\upconversion\error-messages-en.properties
This file is called from UpConversionPackageDefinitions from line 46 with setErrorMessageBundle. The resourcebundle includes the locale which currently only works for en locale.
Suggested fix would be to include (or just rename) error-messages.properties in the package, in that way the package is no longer depending on the locale settings.