Connection to storage server failed. (already gone through recommended fixes)

[ura_soul]

i have completed installing roundcube mail on my fedora 22 server here, using mariadb, nginx and php-fpm.
when i view the login screen for roundcubemail and enter credentials, there is a delay of over 30 seconds and then i see: "Connection to storage server failed."

having researched this issue and attempted the fixes suggested by others in forum threads, i still haven't found the cause of the issue.
i have looked in the logs and see nothing being created in the roundcube log when this occurs (or any other logs).

i have gone through the process of testing the mail services by logging in to them via telnet. there were no obvious problems in me doing that and that side of things looks ok.
i didn't originally install the package dovecot-mysql as i didn't know what it does or if i needed it. i still haven't got much of an explanation as to the purpose of that package as compared to the standard dovecot package, but in any case i now have them both installed.

does anyone have any tips on how to resolve this? thanks

[SKaero]

It maybe a SELinux problem, try disabling SELinux and see if you can connect.

[ura_soul]

selinux is already disabled, so that's not the cause here.