magento2 – Magento 2 – Invalid security or form key. Please refresh the page

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.