All i can suggest is performing an XML export & import and try your new module on the original instance if you haven't already done so. If it doesn't work, it indicates there is an issue with the configuration on the module somewhere. You could also include the XML here so it can be reviewed
A quick google doesn't indicate that mailto links can be disabled for specific URLs via the browser but this could be a cause if possible. I'm not aware of any properties that would impact this, although it shouldn't be too difficult to track what properties have been modified on the first sandbox by looking at the sys_properties list and sorted on updated. My PDI allowed them to work without having to update any system properties
