The error checking under location editing prevents valid locations from being edited and saved.
An example of this, is the default php location that is created with server. If you open that location to edit from Vmin/webmin, you cannot save the location, even without changes.
I am sure there may be a good reason for this, but I suggest that the answer may be to simplify the location editing process, to make it more flexible for advanced use cases, since I have found I am more likely to enter a location formula I want directly, without any wizard help, especially when I know what I want, but not how to use webmins controls to achieve it. I'd rather paste in or type it directly without asstistance. Just a thought...
But the saving thing is a bug. Thanks!