I recently had a site starting generating CSRF Errors whenever a user tried to log on. Coincidently it coincided with an update to the server that I had made.
After a lot of investigation and rolling back the versions (with no change to the error), I discovered that the disk space available to the server was almost full. As it was running on an AWS instance, I increased the disk space and the problem immediately went away.
I am posting this for future reference.
Regards
Craig