Unfortunately, since I'm not experiencing this issue, there's little I can do to troubleshoot this to see what the problem is, so I'll just tell you what I would try to do if this happened to me.
I thought of something after seeing @renojim's recent post and that's to try to connect the Roku to a different outlet to make sure your outlet isn't the culprit because if the Roku isn't getting enough power, it can cause crashes to occur, which is more usual for the Express model (with a faulty USB/Power supply) than an Ultra.
Try to keep manually updating your Roku because maybe an update will fix the issue. Are you using your Roku to watch one channel app? Because perhaps it's the channel that's crashing the Roku after the update. You would need to log out of the channel, uninstall the channel, restart the Roku, and re-install the channel.
Another thing you can try is to go into Theme in settings and "Restore default theme" as perhaps that somehow became messed up and is causing the crash when the Roku attempts to go to the screensaver. Even after a Factory reset, if you're putting the Roku back on the same wallpaper and/or screensaver, this could cause the crash to reoccur, so try to use a different wallpaper and screensaver to see if that's the culprit. You can also turn off your screensaver for a while to see if your Roku still has this issue. If this is the culprit then a future update should fix that, but let Roku know by posting about it.
Before doing a manual update, you can go into "Advanced system settings" and do a "Network connection reset" if there is something with your network connection that causes the Roku to crash. It shouldn't be your WiFi connection, but it wouldn't hurt to connect the Roku directly to the router if you can with an ethernet cord. Also, turn off your router and modem for a few minutes and then turn it on. Then try to manually update the Roku as perhaps doing this could help resolve the issue.
You could then try to remove all your channels (write them down so you know which ones you had), restart the Roku, and then re-add them. First, only add the one channel you want to watch and see if your Roku crashes, and if it does then remove that channel and add a different one and try to watch that one. That way you can pinpoint if a channel app became problematic after a bad update as that may be the culprit instead of your Roku.
If none of this works, one thing you can try, which you would need to do before returning the Roku anyhow, which goes beyond a Factory reset, is to remove the Roku from your account from the Roku website, load the Roku, and then re-add it to your account (like when after you bought it).
If nothing solves your issue and it's still under warranty, you can exchange it with another Roku Ultra, which might be the best thing to do since you can easily transfer your account to it. To learn more about how to do this just go to https://support.roku.com/category/4403790058903 and select "Warranty & returns."
Anything can cause Roku issues from a bad channel update, to a router issue, to a power surge which is why I use my Roku on a battery backup. When I had a bad OS update with my Roku Express due to my router acting wonky, my Roku device just let me know it failed the update and allowed me to repeatedly try again until the issue was resolved, so Roku is usually good about Roku devices that don't update right. But sometimes the update itself can cause havoc with a Roku device, yet usually, this causes others to also have similar issues that can be specific to your Roku model. This doesn't seem to be what's happening here else others would have commented about having a similar issue, (update) but this update is new and it may take a while for others with your device model to receive it.
I'm surprised @renojim that your Roku hasn't yet been updated to v12.5.5, as I always assumed the newer models got the update first and then it would reach the older Roku's, but my Ultra got the update yesterday (1/16/24). All I can suggest is to keep trying to manually update your Roku Ultra. (Update) But perhaps the reason your Roku isn't updating is because there are issues with newer models with the OS update, which is what may have happened to @Mannnnnnnnnnnny, and Roku is trying to work out the kinks before sending this update to your model. If that's the case, then a newer update should fix this crashing issue when it rolls out.
