When trying to access roundcube via webmail you receive an error below. You receive it even if there is sufficient disk space available on the server & assigned to the hosting/email account.
Connection to storage server failed.
Fix to be applied is by performing cPanel updates on the server & restarting mail service.
Login to your server via SSH as root & execute the command below.
Once cPanel updates are completed, restart mail server exim with command below.
service exim restart
Now, try accessing roundcube via webmail & it should work. The above fix can be applied on your own, only if you have your managed VPS hosting OR Dedicated server. On a shared hosting platform, contact your web hosting provider.