Based on the latest comment, I would say it's indeed a server issue. The server announces support for CRAM-MD5 authentication method, but when the client actually tries using it, it fails. The server should be either fixed or configured not to announce CRAM-MD5 support.
Rebex IMAP prefers CRAM-MD5 to ClearText because it's slightly more secure. To work around this issue, you don't need to modify any source code, just call the following variant of the
Login method to force ClearText authentication:
imap.Login(userName, password, ImapAuthentication.ClearText);