I think this thread (http://osticket.com/forums/showthread.php?t=8166(http://osticket.com/forums/showthread.php?t=8166)) was closed too early, as there must be something else to the problem besides just mcrypt: I'm running osTicket on my Synology DS-412+, PHP v5.3.10, and getting the same IMAP error whether selecting IMAP or POP retrieval... but the mcrypt module IS enabled in my PHP settings.
Synology support forum has a thread on this same issue: http://forum.synology.com/enu/viewtopic.php?f=20&t=44981&p=213174#p213174(http://forum.synology.com/enu/viewtopic.php?f=20&t=44981&p=213174#p213174) - no resolution yet.