External monitor keeps staying in ‘Entering Power Save Mode’-loop

I've got a Dell U2515H (production month: June 2016) connected to my Macbook (early 2015) with Apple’s USB C digital AV multiport adapter using an HDMI cable. I’m running the latest software (macOS Sierra 10.12) and latest adapter firmware.

After ten minutes the Macbook goes into power save mode, the monitor shows the message "Entering Power Save Mode" and then the screen turns completely black (backlight turns off).

This is all fine. However, after a few minutes the backlight suddenly turns on and the "Entering Power Save Mode”-message pops up again for a second or two, after which the backlight directly turns off again, a few seconds later the backlight turns on again, the "Entering Power Save Mode" pops up for a second or two, the backlight turns off again, after a few seconds the pop up message appears again, etc. - the monitor keeps in this loop until I press a key on my keyboard and then I see the normal desktop screen again.

This sleep cycle issue is pretty annoying and probably also not good for the monitor (which otherwise actually works great). What can be done about this?

I already tried using different cables, restarted the Macbook, did reset PRAM and the SMC-controller, power cycled the monitor, turned on DP 1.2 (monitor setting), and turned off DDC/CI (monitor setting) but nothing helped so far.

Thanks in advance for your help!