Getting 'IP blocked' warning on login page, though it's not

Just recently, I started getting this warning on the login page:

“ERROR: Login failed because your IP address has been blocked. Please contact the administrator.”

But the IP is NOT actually blocked, I can log in just fine. Nor is there any reason it should be blocked.

I did have “Instantly Lockout Invalid Usernames” checked, but no user name had been entered before getting this warning. I unchecked it anyway, to see if it made a difference, but it does not, still getting the same bogus warning.

There are no IPs listed in the “Permanently Blocked IP Addresses” area.

I am on a Nginx server.

We have many contributors on our site, and I’m worried people will think their blocked out when they are not.

What is causing this, and how can it be fixed? Thanks for any help.

https://wordpress.org/plugins/all-in-one-wp-security-and-firewall/