If someone see the "error" on the corner, it means his/her IP has been blocked.
Why they're being blocked? Many possibilities:
- Too many attempts of login using wrong password or username
- Too many 404 errors being generated by the IP
You may check your log to see if someone being blocked because of using wrong password/username.
On your log you can also see if someone being blocked because of 404 errors. Fixing 404 issues can be a hard thing because there are too many possibilities which can generate 404 errors:
- You're using a theme or plugin which is not compatible with the change wp-content directory feature.
- You're using a theme or plugin which is not properly written.
- apple-touch-icon issue
- you've just moved a page/image/file but someone still link to the non-existing resource
- you've just deleted a page/image/file but someone still link to the non-existing resource
- you've just changed your permalink configurations
- etc ...
So, what should you do?
You need to find out what kind error the user generated which cause the blocking.