The login fails if you try to access to the application using /jenkins instead of /jenkins/. You can workaround this issue editing the file at
/opt/bitnami/apps/jenkins/conf/httpd-prefix.conf and modifying this line:
RedirectMatch ^/$ /jenkins/
Restart Apache after that
sudo /opt/bitnami/ctlscript.sh restart apache
Another way to access to jenkins is to use http://yourIP/jenkins/ instead http://yourIP/jenkins. I could login into the application using user/bitnami in the login panel.