This is a the same bug as reported by Joe here (wanted to comment but couldn't).
Modifying Unix user ..
.. Unix user failed! : Cannot write to directory at ../web-lib-funcs.pl line 834.
Changing MySQL password ..
Updating Webmin user ..
Saving server details ..
Re-loading Webmin ..
I get this error when attempting to change the administration password in the edit_domain form. It occurs on all domains. I am running 3.0.7, all other modules are fully updated