Can you login to the serial console with the same credentials?
Could it be that you have special characters in your password (like ' & # " ß)? I think there was in the past something that in the WebUI such characters were 'escaped' and stored incorrectly. If you can change the password, try to change it to a simple one to see if that may be the case.
If that is the case, or to get updated anyway, you may try to upgrade to the latest version for the 3600: 6.4.4.23.