I am having the same issue after upgrading to 1.8.
Auto-cron is fetching the mail, but fetching mail by using task scheduler and cron job is not.
No errors are reported in the event viewer, but when I pipe the result of running the job to a log file I see
"kwaheri rafiki!"
osTicket Version: 1.8.0
Mail Sending: phpMail
Mail Fetching?REceiving: IMAP
PHP Version: 5.5.1
Operating System Windows 2008 R2