@jell yes, sorry for having described my problem poorly, it is all in lowercase indeed (downloaded_images, rom folder, gamelist folder and all the path references)
@jonnykesh you nailed it... just wow... i thought gamelist files were only located in the specific /gamelists/xxx/ folder and never noticed there could be one inside the /roms/xxx/ folder as well. I always worked with the former one. Now i feel stupid. I guess having spent an entire day on that is punishment enough. While i was also thinking of a possible second gamelist file, i initiated a file search via WinSCP from root directory and it would only show me the ones located in ./opt/retropie/configs/all/emulationstation/gamelists/xxx/ I now figured the one in the /roms/xxx/ folder is optionnal but will take priority over the one in /gamelists/xxx/ folder.
Thanks guys!