The UID error is an outlook error. It happens when Outlook is trying to syncrhonize the e-mails.
Each e-mail is given a numerical UID number, and all new e-mails must have numbers higher than this. So, there are times when an e-mail is deleted off the server? Or through webmail, or locked in some fashion, and when Outlook tries to synchronize this e-mail, it get's the UID error.
This is not something that is unique to Icewarp, but is a general issue with Outlook. Below is Microsoft's webpage about this error.