Issue Adding Second Interface

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Having issues with Endian Coummunity 2.5.1 adding a second network interface.

I want to add a second link and set it as a fail over so it connects to a different network on another network card, I can add all the details in perfectly fine.

When I submit the page just goes white, I don't get any error messages or notifications and it just won't add the second interface in.

I have tried running updates but it seems the box is upto date using the latest stable patches.

Does anyone have any clues? I really need to get the failover working asap.