Ok, another update. This is the video of issue I'm running into:
Setup: My retroelectronic supergun is connected to an arcade powersupply which provides both 12V and +5V (no -5V) to my MVS 1FZ w/ unibios 4which has a battery mod. I tweaked so that it delivers +5.1V to the supergun. I then checked with the MVS1 powered on running the NeoSD (No SD card inserted) what the power draw is of a regular game when it's running. C3 on the Jamma board measures 4.2V Jamma components side, pin D4 measures 5.1V Jamma components side. Regular carts seem to work flawlessly, checked 3 random games. On the control panel side I connected buttons Up, down, left, right, A,B,C,D, Left Player start to the supergun terminal blocks, same for Player 2. Dip switch 7 is set to ON (Freeplay) and the backup ram has been wiped. All tests seem to be fine. I tried switching out different BIOS versions, even stock.
SD card: I am using a generic MicoSD card 16GB, formatted FAT32. Roms are validated using the NeoValidator 1.06. Couple of romsets missing but the rest is fine.
Now, I noticed that the card is locked when I try to wipe it. Tools like diskpart etc. seem to fail. The card is write disabled apparently and no way to delete it(?). It was relatively new but cheap. Since the NEOSd writes back to sd card, this would go wrong. Is it enough to trigger a reboot? Maybe, hoping somebody can tell me. Tried putting it in arcade mode with a flashed game on the card, no succes.
NeoSD MVS Pro: boot to menu, a+d+start in game menu. Arcade = no. Backram set to No but have tried other options. Nothing that causes me to get trapped in an endless bootloop. Flashing games works ok. When I start a game, the NeoSD triggers a reboot straight to Menu.
SUPPORT: Tried opening a ticket over at terraonion but wouldn't you know it, it is impossible to register because nobody activates my account. I guess NG.com is the way to go then!