Can't get my own boot videos to play off USB

[RustyMG]

Hi all ,
Updated my setup on a PI3b from 6.2 to Recalbox 7, again booting from microSD and all the games, overlays etc being on the usb stick.
Everything is working great (well, apart from no MSX Turbo games will run, despite MSX and MSX2 ones all being fine), but, I just cant get it to play any of my own "boot videos" stored on the usb stick.

They are in the correct folder, ie recalbox/bootvideos, and I've amended the flag on the config file to only look in the shared / external folder and to play until finished, but nothing - no videos play.
I've tried 720p, 1080p, mkv, mp4 files, but none play - just get the black screen until EmulationStation starts booting.
Any suggestions ?
Am I doing something really silly?

Thanks in advance, and thanks to the developers for the latest release - it's the best around.
I have no idea why anyone would want to use ANY other Pie systems.

[acris]

check recalbox.conf to make custom information en bootvideos

[RustyMG]

Hi,
As mentioned, I have. That's what is puzzling me.
Settings I have to play full video from my usb are the following in the file: /recalbox/share/system/recalbox.conf

system.splash.length=-1
system.splash.select=custom

Yet, none play. Any suggestions?