1) Dovecot should be logging to the file /var/log/maillog. But you might want to have a look into the dovecot.conf file if additional logging can be activated there.
2) Not easy to say. First try to find the file were dovecot logs to. It might even be a hardware problem.
3) Using yum frequently is fine and recommended.
4) /etc/init.d/ispconfig_server restart
But your problem is most likely not directly related to ispconfig, as ispconfig does not write configuration files for dovecot.
5) Use yum and additionally update to the latest ispconfig version.