Dell U2515H: Native resolution distorted
I recently purchased a Dell U2515H monitor. I want to use it with my MacBook Pro 13'' 2012, non-Retina, 2.5GHz. According to the tech specs, this MacBook can drive a single 2560x1600 pixel display (http://www.apple.com/macbook-pro/specs/).
However, for me, it fails.
I tried two different DisplayPort-cables, one came bundled with the screen. Running the display at 2048x1152 pixels (the second highest resolution) works fine. For the full resolution, OS X preselects 59,956Hz, I can't change that.
I tried under Bootcamp Windows 10 on the same machine, and the display works fine under 2560x1440.
Any idea what to do?
MacBook Pro (13-inch Mid 2012), OS X El Capitan (10.11.1), Samsung SSD 850 Evo