Hi, @Julos1er. That's odd. If you're still experiencing this issue, I highly recommend Wiping the cache partition on your S21 Ultra. This will not erase any essential data, and is a useful step to follow after a software update. It effectively gives the software a fresh start, and you can read more about this in the thread below:
https://eu.community.samsung.com/t5/galaxy-s21-series/cache-partition/m-p/6363335#M47139
While I'm surprised that Wiping the cache partition has not helped on this occasion, @Motty2012, it just means that we can look at other possibilities. 👍
For anyone who's followed the steps above and still hasn't resolved the issue of their phone restarting randomly, the next step would be to pop your phone into Safe Mode. This will disable any third-party applications, so that we can determine whether these are causing the issue. To do this, please press and hold the Power button until you see Power off, Restart and Emergency > Press and Hold the Power Off icon until it shows a Shield > Tap the Shield. After this, the phone will start in Safe Mode.
If the restarting is fixed/not evident whilst in Safe Mode, then it suggests that it is being caused by a third-party app. To resolve this, exit Safe Mode, then begin uninstalling apps you've downloaded one by one (restarting the phone between each app uninstallation) until the issue fixes itself. Once you've identified the troublesome app, you can reinstall the other apps via the Galaxy/Google Play Store.
Please let me know whether this helps.
