Thought I'd reply here as well as I've been having the same problem: black areas combined with flickering/pulsing.
When exporting the video, the upper left corner also reads "2 operations in progress". You have to wait for the 1st one to finish (not sure what the 1st operation is, though), and only then does it read "Export slideshow as video".
I've always been using the same template: 24fps Timelapse.lrtemplate (after editing my pictures), never had no problem with it. After switching back to version 9.2.1, the problem was solved.
As much as I'd like to use the latest version of Lightroom, it woud be nice to have this problem solved before I update again to 9.3
im having the same issue also. fustrating! ive tryed both my cameras canon 80d and Sony A7II and reset both cameras. always was shooting RAW. ive also tried on my two computers imac 5k late 2024 and macbook pro 2017. same issue. i also tried new SD cards. i noticed this started happening around March of this year. i've never had an issue prior to March 2020. - gil