Nice. The progress is the word "Failure".
This message is the result of one check ( see /usr/local/captiveportal/index.php - at the bottom ) : if the user+pass are in the local user list, access is granted.
A second check is done if you checked "Allow only users/groups with "Captive portal login" privilege set" on the Captive portal settings page.
If these test(s) are ok, you are logged in. If not, a "Failure" is thrown out.
So, if you are sure the user and password are ok when entered, and you are sure they are in the local user manager list, the conclusion is simple :
Consider your system broken - probably a hardware (disk) failure.
If a login works - and after some time (hard or soft time out ?) the user can't login anymore, then something is not working as it should.
Btw : I'm replying knowing that you did not mention anything about your setup, so I consider every setting is set (kept to) to default (value). It is of course possible that a user logs in, and after time out he can't login anymore for the rest of the day. Or something like that.