I have this error when creating a new project on 2021.12, and after laborious disabling and resetting various preferences, I think I've narrowed down the problem.

Like most editors, I like my timeline buttons configured a certain way.  At the university where I teach, I've written tutorials referring to specific buttons that are in specific places, so it's even more important.  I'm in the middle of updaing my machines and started having the SETTING CLASS NOT REGISTERED error as I was testing things out.  I followed the procedures here but they did not help:

https://avid.secure.force.com/pkb/articles/en_US/Error_Message/Exception-SETTING-CLASS-NOT-REGISTERED

So I started with a fresh user and changed my settings, first all at once, then one by one.  What I found was that attempting to add a Segment Mode (Lift/Overwrite) red-arrow button anywhere on the timeline button strip will cause the error.  Remove it (replace it with a blank button or any other function), and the error goes away.  I initially thought it was a problem with a particular spot on the button strip, but I can add other buttons to that spot or any other without causing the error.  I tried adding Segment Mode (red) to other available buttons on the strip and this also caused the error.

This seems like a genuine bug, and it's a kick in the pants because I need that button so that it lines up with my tutorials.  I'm on MC 2021.12, Mac OS Big Sur 11.5.2 on a late 2013 Mac Pro.  Any insight from the peanut gallery, or should I call support?  Or should I roll back to 2012.6?

I too had this problem. In the command palette there are 2 places where you can get the segment mode: the edit page and the smart tools page. I think if you take the button from the smart tools page you clear this error message, 

I'm not sure what the differenece is

St.John

That worked!  Thank you so much St.John.  This is what having forums is all about.  I had been pulling the button from the Edit tab of the command palette every time I added it.

Avid employees: this appears to be a very specific bug.

Thanks a lot !

I also worked for me !

And for me!

Hi,

This fixed my problem with this error warning, on one of my computers that always seems to have 'user' issues every time I uninstall and upgrade to a new MC version, even after creating new user settings.

Thanks,

This fix worked for me as well, thanks.

Worked for me too, THANKS! 

HALLELUJAH !!

Thanks