After some testing, i discovered it happens to me if i have the enhanced resolution option and integer scaling on at the same time. If i turn off integer scaling, i get full 4/3 (game ratio is "Square Pixel" in emulationstation).

Is this working as intended?

Odd, it works perfectly for me I've used it for all my systems.

@steveb83 will try it out thanks

Just a short notice:

Could also write the Image onto another microSD-card which afterwards booted flawlessly.

Well if anyone read that thread : please don't do that !!! All your problems come from your downloading of random BIOS files : this files are already included in your 0.78 romset in the correct version needed by mame 0.78. Which is not the case of the ones you downloaded which can come from any mame version. If those files are not in your romset it's not a good nor complete one And it's pretty clear in the wiki easy arcade that those BIOS files are in the romset. So downloading other files and using clrmamepro is totally useless AND creating problems, like you experienced Also there's no need at all to modify recalbox.conf like you did : mame078 is the version used by default in recalbox