I just ran into this same error today. No errors in the logs, no recent changes. no clear cause of the issue. USB drive checks out fine. All dockers and shares are fine.
I just ended up locating var.ini and doing "ln -s /var/local/emhttp/ /usr/local/emhttp/state" which seems to have fixed it, but it's really mysterious why this was necessary or if this is even the proper fix.