Welcome to the Lightroom Community. I'll help you figure this out. Before anything else, Lightroom Classic 12.2 is available via the Creative Cloud desktop app. Update & report if the issue exists.
If the issues exist, try these steps:
- Use images with uniform dimensions (only landscapes or portraits) in the slideshow as a test.
- Go to Lightroom > Preferences > Performance > Use Graphics Processor > Set it to Off if it is on Custom or Auto. If this shows you any differences, please check this article to troubleshoot further: https://helpx.adobe.com/lightroom-classic/kb/troubleshoot-gpu.html
You can also try to clear the DynamicLink Manager (it handles video playback in Lightroom) by following these steps:
For macOS, delete the following on your system:
- /Users/<UserName>/Library/Application Support/Adobe/dynamiclinkmediaserver
- /Users/<UserName>/Library/Application Support/Adobe/dynamiclinkmanager (if exists)
Delete the following to clear Video Cache:
- /Users/<UserName>/Library/Caches/Adobe/Lightroom/Video/Media Cache
- /Users/<UserName>/Library/Caches/Adobe/Lightroom/Video/Media Cache Files
Restart your computer, and then try to create a slideshow again, and let me know if that works!