cMBP 13'' m/2012: 2560x1440 over DP fails

[senthor]

Hi everyone,
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 2048*1152 pixels (the second highest resolution) works fine. OS X preselects 60Hz, I can't change that.

Any idea?

Thanks!

 

[senthor]

I tried a reboot, I tried another user account, I tried a different port on the monitor...
Is this an issue of my MBP or of the screen? Any idea how to find this out?

 

[senthor]

Just tried Windows 10 on the same machine. (The really obvious solution). Works flawlessly, 2560x1440@60Hz.
Will try a fresh OS X on external drive...