<ipSecurity> issue?

Question

We restrict access to a folder in our web using <ipSecurity allowUnlisted="false"> in our web.config file.

Code portion:

<location path="unicol">
<system.webServer>
<security>
<ipSecurity allowUnlisted="false">
<clear />
<add ipAddress="xxx.xxx.xxx.xxx" allowed="true" />
</ipSecurity>
</security>
</system.webServer>
</location>

Up until 8 days ago it all worked fine, at which point whenever someone tries to access the restricted folder we receive the message "The page cannot be displayed because an internal server error has occurred."

We have tested and the when we remove

    <ipSecurity allowUnlisted="false">
      <clear />
      <add ipAddress="xxx.xxx.xxx.xxx" allowed="true" />
    </ipSecurity>

from the web.config, there are no errors - but of course there is also no access restriction.

No updates were made to the web.config prior to the errors, so my assumption is that a server setting may have changed? Is my assumption correct? If so, what might be causing this?

Thanks,
Mart