A client currently has an "under construction" page while they are making changes in the backoffice. However they have sort of a profile page on another website which they want visitors redirected to instead.

So I am trying to make a 302 redirect rule of all urls except everything under /umbraco, so they can still access the backoffice. The redirect rule technically works, except it makes the backoffice login page blank white, with the console error:

Uncaught ReferenceError: UmbClientMgr is not defined at Application?umb_rnd=fffffffa11a040bc:1 at finish (lazyload.js:108) at HTMLScriptElement.finish (lazyload.js:162)

My redirect rule looks like this:

<rule name="302 temp redirect to external website" stopProcessing="true">
      <match url="^((?!umbraco).)*$" />
      <action type="Redirect" url="another.site/client" redirectType="Found" />
    </rule>

The login page works fine without this redirect rule. The site is running on Umbraco 7.3.6