Virtualmin IP problems

I have CEntOS5 Virtualmin GPL setup which had a few top level servers, sub-servers, and aliases. They are split between 3 IP addresses. For some reason when I try to add another to the default shared IP the following error appears:

Failed to create virtual server : Invalid master IP address : none

I also tried adding the new virtual server to one of the other existing addresses, I get this error:
Failed to create virtual server : The IP address is already used by virtual server

The same error occurs when I try to make additional sub-servers, but aliases are created fine.

I’m not sure what has changed in my configuration, but these operations worked fine in the past. Has anyone else encountered this problem? Is there any way to turn on more debugging information to narrow down the cause?