OS: Windows; IceWarp Server Version: 11/10

The UID error is an Outlook error. It happens when Outlook is trying to synchronize the e-mails.

The UID is a value that the IMAP server and client agree upon that identifies a particular message.  The message indicates that the client is now calculating a different UID for the same item.

This is not something that is unique to Icewarp, but is a general issue with Outlook.

Workaround 1

Close Outlook, and any other IMAP client connected to the account.
Login to the webmail interface as the user.
Create a new folder called 'backup'.
Open the folder that is affected and select all of the messages, then drag and drop them to 'backup' to move them.
Go to 'backup' and select all of the messages and drag and drop them back to the original folder.
Delete 'backup'.

Workaround 2

Recreate the account on server side (after making sure you backed-up the user's e-mail folder on the server)

Stop Outlook.

Restart it and add the new account to Outlook.