Monitor suddenly gives "no HDMI signal from your device", except when resolution is set to 1280x720

I have a 13" MacBook Pro early 2015 and recently bought the 4k Dell U2720Q monitor, for this I bought a Mini DisplayPort to HDMI cable (Mini DisplayPort 1.2 to HDMI 2.0 cable (4K 60 Hz))

The first day this worked fine and the monitor worked, and at full resolution. After a day the monitor suddenly started giving the error "no HDMI signal from your device".

What I have tried to fix it:

• reset NVRAM (unsuccessful)
• reset SCM (unsuccessful)
• unplug; reset; reconnect, etc (unsuccessful)
• enable display mirroring (successful only when optimised for dell monitor. Worked at first but after two times also didn't work anymore)
• changed the resolution of the monitor in settings, selecting all options and seeing what worked, only the 1280x720 works

The only thing that worked for me was setting to resolution of the display to 1280x720. There is also one lower resolution (1152x648) but that doesn't work either.

Is there a way to fix this issue so that I can use my monitor at the correct resolution again?

The strange thing to me is that at first the monitor worked without problems

Thanks in advance for your time