It looks like there is indeed a bug – and thanks to all the command output you’ve provided above, Jamie was able to figure that out. It was a problem due to the language of your system, Webmin wasn’t handling it not being English properly.
This issue will be fixed in the next Webmin version.
In the meantime – the only ways you can get around that issue are:
Set your system to use English for it’s language, and then restart Webmin.
Disable that script until after the next Webmin version comes out.
Have a good one!