Hi, I can confirm this is also afftecting my server.
I have 1 client unable to login to IMAP via webmail, or traditional mail client.
All other imap accounts appear fine.
I have checked the logs while trying to login to the account in question, and it errors with:
imapd: authentication error: Input/output error
I dont think its password related, as you can access webmail, but not select a webmail client to use.
The account in question can access the webmail menu, but gets login errors reported by squirrelmail when trying to select it.
The password for the account has been reset many times, while testing with thunderbird, and the imapd and exim services have been restarted.
I have updated to the latest version with the same issue.
WHM 11.11.0 cPanel 11.15.0-R18264
CENTOS 4.5 x86_64 on standard - WHM X v3.1.0
Support ticket raised: 227112