The ‘Oops’ page is Internet based, i.e. your computer has a Internet connection, and when you entered the URL for the extender Admin pages e.g. mywifiext.net this took you to the Internet rather than the Extender page.
The typical reason for this is that the computer is associated with the base station and not via the Extender, the base station has no knowledge that the Extender would normally intercept the mywifiext.net URL and simply passes it on to your ISP DNS servers, which leads to that Internet page.
So, you need to scan for networks at the PC, connect to the Extender SSID and enter the correct passphrase, then try that URL again.
