Thanks, that seems to get rid of that console error. Now the games just hang, will do some more troubleshooting :D

Solved?

The problem was that the pi4 has 2 "connected" outputs (given by xrandrs output), this means that the MODE_CUR array is increased by one (HDMI-1 HDMI-2), and MODE_CUR[5] becomes aspect ratio instead of being the refresh rate, my solution basically looks for /primary/ instead of /connected/... but no clue if that will mess things up for others.

runcommand.sh
runcommand.sh.diff

In 2.0.10, it looks like a hint was added (SDL_HINT_GAMECONTROLLERCONFIG_FILE) to point to a user chosen 'gamesdbcontroller.txt' file. It looks like PPSSPP doesn't use this mechanism to load the file, but it might affect how SDL2 works when the game controller API is initialized by the emulator.

I'll see if a recent source build for PPSSPP triggers this on a RPi, it could also be something that's changed in the emulator since the last binary build in RetroPie.

@BuZz editing /opt/retropie/configs/all/runcommand.cfg did the trick, thankyou! :-)