universal control stopped working

Ensure that both your iPad and Mac computer have been updated to the most recent release of iPadOS and macOS. Universal Control has only just transitioned from a “beta” release to a full production feature.

Universal Control: Use a single keyboard and mouse between Mac and iPad - Apple Support

Having updated both iPad and Mac, if you are seeing unusual behaviour or broken functionality, force-restart your iPad:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

If you continue to experience difficulties, you may need to seek assistance directly from from Apple Support. You can contact the Support Team using the Get Support link at top-right of this page.

Alternatively, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:

https://apps.apple.com/gb/app/apple-support/id1130498044

I feel your frustration. Having performed what you have - without success - a Factory Restore is genuinely the best way forward.

The Factory Restore is not as painful or as onerous as you might initially perceive. Providing you have created an iCloud or iTunes backup of your iPad before proceeding with the Factory Restore process, you will simply restore your pre-existing backup to your iPad at the end of the procedure.

The procedure is straightforward. Nothing will be lost - other than a couple of hours effort.

Restore your iPhone, iPad or iPod to factory settings – Apple Support

Restore your iPhone, iPad or iPod touch from a backup – Apple Support

Restore all content to iPad from a backup - Apple Support

You’ll need access to either a PC (with iTunes installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of MacOS).

Download and use iTunes for Windows – Apple Support

Update to the latest version of iTunes – Apple Support

Basically you're saying that I should restart the computer and iPad. I've tried that (a few times) and I've turned Universal Control off and on a number of times in every possible configuration. If I were possible, I would have pulled out the cartridge, blown into it, and reinserted. I've done All The Things short of a re-install of the whole OS, which I will not do.

And it still doesn't work. It USED TO work, and it was awesome. Really useful. I used it all the time and it made me more productive. But now it's just burning cycles and making me frustrated.

Has anyone got anything more useful that restarting?

Same problem. Driven me mad for 24 hours. Caused by iPad update to 15.5. Solved by forcing Mac update to 12.4 and a couple of restarts. Be good if Apple said in the update information that these updates would break UC if not done together on Mac and iPad,

I figured it out. The next restart after my post above, and suddenly it works. I guess an incantation to the gods was what I needed to do!

I’m delighted to learn that the problem is now resolved.