Very good, had no issue for the whole process to complete. I thought about soldering but then lazyness took over

Could not find one easily in France. Works with a standard USB cable but not mistake proof

Looks like the subtitle format is not compatible with your mobile client and needs to be transcoded server side. I’d check from the jellyfin admin console while watching incriminated content. Also you could try findroid with MPV player as it is more compatible