Connecting macbook pro (2019) to Samsung monitor turns both laptop and external monitor screen black

I tried to connect a Samsung monitor (SyncMaster 226BW) to my Macbook Pro (2019). At first it seemed to work for a few seconds, but now every time I plug it in both the laptop screen and the external monitor screen go black. Sometimes the laptop blinks for a second or so. Its driving me crazy! What can it be? I connect my laptop with a new VGA dongle to a new VGA cable (checked with another cable too).. I can't do anything in settings on my mac, because every time I plug in it turns black (it doens't turn black when it's only connected to the cables and not the monitor). Someone else knows this problem? I searched quite a lot but couldn't find the answer. Thanks a lot! :)