Ok, so its NOOBS to blame and i shred the system twice trying to update it. Always ends with formating the SD-Card and loosing the old raspian... but before shreding the normal rainbow screen appears and then NOOBS is starting. So, yes it was NOOBS... Think on the other SDCard i did an install with ...

Ok drgeoff, do you know how to update or upgrade the files in the boot process? I thougt of some wrong software for the GPU but expected the update / upgrade to do that trick with the bootfiles and not only with raspian? I tried to excange the bootcode.bin and .elf Files, but that dont work for me. ...

Ok, thanks for the Linux skills i'm lacking... the cat /etc/os-release yields pretty the same output as your example. Raspian 9 (stretch) on both installations. Strange to me the booting install on 3B+ respond to uname -a with a 4.14.34-v7+ whereas the install only running on 3B tells me 4.14.37-v7+...

I have roughly the same problem on an working image on a Pi 3B i want to use with a new (and working) 3B+ but not even getting the rainbow screen. i have made sure no usual problem with power source and video signal exist. In fact i have one installation wich is working fine on both Pi's after doing...