I have recently upgraded from 2.3.7 to 2.4.3 and since the upgrade I am seeing

> 
> "Invalid security or form key. Please refresh the page."
> 
> 
> 

on every page in admin. (frontend works fine)

Most things in the backend still work even with the error present apart from the ability to add products to a manual order in the backend.
I have disabled all 3rd party modules and set max\_input\_vars to 200000 but it doesn't solve the error.

I have cleared all caches and removed all generated code etc and also run setup:upgrade. I have also checked to ensure cookie path and domain are valid. But still the error shows! Anyone have any other ideas? Or how I can debug further? Thanks.