Using same logon credentials, one user can logon, other user cannot

Anyone seen this weird scenario:

The WP site has an established user/pwd.
One person goes to site.com/wp-admin and can logon successfully using that user/pwd.
Second person, different location, using same user/pwd cannot logon. Receives a “ERROR: The password you entered for the username XXX is incorrect. Lost your password?”

Additional workaround was tried:
First person, who can logon, creates new user/pwd for 2nd person.
2nd person receives email from WordPress detailing new Username and Password and link to http://site.com/wp-login.php

2nd person selects link in email, enters user/password and gets an error message: “ERROR: Invalid username. Lost your password?”

Other options that have been tried unsuccessfully:
– remove cookies
– clear cache
– different browsers

So far, it looks like WP has a convenient (but not fully understood) lockout mechanism. Just never know when it will happen or how to overcome it???

Any insight would be most appreciated.