Either the username was not recognised, or the password was incorrect

Hello to all of you! 

I am using freeviewer with a non-paying account, with three associated users and two addresses. One of them works perfectly, but in the other one, although I use the same admin as always and the same password that worked before, now I get the message "Either the username was not recognised, or the password was incorrect" and I can't change the password and I don't know how to solve it... 

Has anyone had the same problem and solved it? 

Thank you!