Magento 2 Display error message after update 2.4.3: Invalid security or form key. Please refresh the page.  While we had this problem we were unable to export CSV table rates.

If you find the following in your system.log:

main.WARNING: Session size of 265085 exceeded allowed session max size of 256000.

The problem is the session size.  To increase it, execute the following command 

bin/magento config:set system/security/max_session_size_admin 512000

bin/magento c:f

This will double your session size and fix your problem.


Another way to do it - using admin interface:

Store -> Configuration -> Advanced -> System -> Security -> Max Session Size in Admin 

Enter: 512000