No signal on second monitor (BENQ GL2450)

I want to use two external monitors on my MacBook Pro (Retina, 15', Mid 2014, OSX El Capitan) but on the second one (BENQ GL2450) I get the message "No signal".

Both monitors are connected via HDMI-to-MiniDisplayPort cables but only one of them (BENQ GW2250M) seems to work. When I click on the display icon in the upper right of the screen, the second monitor is detected but listed as a TV.

The monitor is not defect and I've already tried several reboots. Going into clamshell mode doesn't solves the problem. The monitor only has a HDMI and VGA port, so I can't use a DVI-Cable...