the problem has already been mentioned in #1244525
the suggested solution was:
This will happen if the system on which you are running
Webmin cannot connect to the internet. The short-term
to go online, make the changes, and then disconnect.
Future releases will include a better fix.
sorry, but this is not a usable fix (at least for me).
i'm forced to connect to the internet through a proxy
only (no other way to get out of corporate lan).
in earlier releases it was possible to change the proxy
settings in the config file, which seems to be ignored
by now. if someone could tell me where to put in the
settings manually, it would be really welcome.