The bank is being used by another member of your Warband

[Magnarok-amanthul (Magnarok)]

There was a blue post about it. There’s a bug so it’s disabled at the moment.

Warband Warbanks are Temporarily Disabled

Along with the launch of the pre-expansion patch (11.0.0), we’ve temporarily disabled the Warband Warbank while we work on a critical issue. This is a top priority and we intend to enable the Warbank as soon as possible. Thank you!