I got this error today trying to log in to an admin account with ‘use admin ip whitelist’ disabled, I’ve had this issue before but couldn’t figure out what was wrong. My IP was not on the logs → screened IPs block list (or in an ip range, which I’m not using).
On v2.2.5 I had a list of blocked IPs from spammers, after looking up more info on these IP addresses (https://whatismyipaddress.com/). It showed that a lot of them are owned by Cloudflare. So it must be due to Cloudflare proxying.
Just a heads up for people using Cloudflare (exluding DNS only mode), this might be causing your issue.
I believe the real IP can be obtained from Cloudflare using CF-Connecting-IP or X-Forwarded-For headers: https://support.cloudflare.com/hc/en-us/articles/200170986-How-does-Cloudflare-handle-HTTP-Request-headers-