Thanks Jan. It's called Open Hardware Monitor and it monitors the temperature of my CPU and GPU. I tend to like to know if they're running too hot and like to make sure that the idling temperatures are suitably low, but it's not essential I suppose. I disabled it and this is what I still get after another restart (native UHD screenshot this time):

As you can see not only is the taskbar still wrong but even the background solid colour has reverted to the default black that I set in Windows personalisation settings. I had set WB11 to make the background copy the accent colour, but this setting keeps turning itself back off again after any restart. Not only that but Object Desktop itself refuses to startup with Windows and it keeps removing itself from the list of startup programs.
I did say that apart from all this I have no other problems with W11, but to be honest sometimes when I double-click on a program icon, the first time of asking it doesn't actually do anything, but if I simply try again it always seems to work fine, which is a bit weird I know. It hasn't been much of a problem, but maybe that's why Object Desktop for example isn't starting up with Windows, until I manually start it afterwards.
The best advice may well be to basically wipe my SSD and start all over again with a fresh installation of W11 and then Object Desktop, etc. but to be honest I've got so much installed on my PC, the vast majority of which runs very well, that I'm not sure I'm willing to do that yet. I don't regret upgrading to W11, because my PC seems to run cooler and faster than before somehow, but when it comes to Object Desktop it is actually not as good.