A few days ago, i received my raspberry pi4b with 4gb ram. I set it up using Raspbian, which worked. I then switched to Ubuntu since I needed some software which is not available for Raspbian. I then noticed, that my Ubuntu Version was too new (19.10) so I tried downgrading to 18.04 with a custom Image. From there on everything seems to fail. I never managed to get any display output at all, even when I went back to the raspbian version I got working previously. I have tried different SD Cards, different Monitors and cables. I have tried adjusting the config file as stated in the troubleshooting guide. I have tried fixing the bootloader. When I run the Pi without anything attached, I get the 4 green led blinks. When I start it with an SD Card the green light blinks randomly, which is expected. I also tried connecting an ethernet cable and can see the lights of the ethernet plug flickering, which is a sign of a successful boot as far as I know. I've tried everything I can think of but I can't seem to be able to fix it. I noticed that sometimes, when installing the raspbian with balenaEtcher it said "1 failed device" and that some checksums were not right. However when reflashing, the error disappeared. As far as power issues are concerned: Using a laptop power supply so that should be enough, and I've also tried others. As said in the beginning, I got the pi working with the exact same setup before... Any ideas?^
EDIT: Pretty sure that it is a hardware related issue as I tried completely fresh SD-Cards. So please move the topic back..