nospam_me wrote:

I've double checked my "Settings ->Format" and have YUV 8 Bit there, with 1440p as the dimensions of my project.

The settings must be YUVA-8bit or RGBA-8bit.
"A" letter means Alpha channel.

IgorBeg
(my current Cinelerra-GG release: Ub16.04_20201031_static)

PS: your nikname is really bad. :-/