Mac Studio: Video Toolbox corrupted and macOS Sequoia reinstallation fails

The “Disk Cannot Be Downgraded” message happens because the Recovery installer treats your current build (even a minor point release) as newer or restricted compared to the specific build footprint on your drive. To bypass this, you must completely wipe the internal volume via Disk Utility in Recovery Mode before reinstalling. 

Reinstalling macOS won't help unless you first erase the drive and make sure not to restore apps or settings from a backup. Plus do not install third party apps which caused the problem in the first place....whatever that app(s) may have been. macOS now resides on a read-only signed & sealed system volume, so the macOS system files are not the problem.

A better way to start troubleshooting the problem is by running the third party app EtreCheck and posting the complete report here so we can examine it for clues. Perhaps afterwards it may be required to erase the system, but at least we can advise you on which third party apps to avoid reinstalling.

How to use the Add Text Feature When Posting an EtreCheck Report - Apple Community

CleanMyMac is infamous for corrupting macOS so uninstall it. But sometimes backup, erase, install, and restore only user data files is needed to fix the damage it has done.

TimDanaher wrote:

Video Toolbox is Apple’s own codecs for HEVC (x.265). Handbrake does use FFmpeg, except if you choose the Video Toolbox option to get x.265 encoding. Then it uses Apple’s own codecs.

OK.

You have quite a number of 3rd party system modification apps. Several of those are firmly "grey market". Avoid anything that provides "clean-up" or "security" services.

The iSubtitle app has actually been crashing quite regularly. I don't know to what extent some of these apps modify system settings. It might be easier to erase the hard drive and reinstall the operating system. Then be much more cautious about 3rd party software.

Something like Handbrake might have an internal log that could give you more information about the problem.

Since these are mostly just apps, you can try creating a new user account on your machine and try it there. If it works, then the problem is confined to your user account. That might make it easier to correct. But if it happens in a new account, then the problem lies at the system level. If you can't track it down with log files, you'll have to wipe and reinstall.

The only app that still shows up on your EtreCheck report from "a few weeks ago" is Splashtop Streamer. That could be the cause. You could try uninstalling that app and see if it helps.

TimDanaher wrote:

Thanks, both... I managed to update to 15.7.9 via software update, but no joy. I just tried running EtreCheck and I get the message: "You are running EtreCheck Pro on a beta version of MacOS. ECP will not work on beta versions of MacOS" ...

You must be using an old version of EtreCheckPro. EtreCheckPro needs to be updated so it understands your current operating system.

I seem to have a problem with Video Toolbox being corrupted. Applications (like Handbrake and SketchUp) that use it fail to export anything. Apps that use FFmpeg (like iSubtitle) have no problems.

I'm not sure what you mean by "Video Toolbox". I'm quite certain that Handbrake also uses FFmpeg.