The server responded - Remote POP3/IMAP accounts are disabled to access via the Webmail system. Only local accounts are permitted for access. As the Administrator use the Webadmin > Config > Mail Configuration to alter this permission setting
Refresh and try again
Check that you are not accessing your mailbox simultaneously in two or more windows. A file-lock is active for the session accessing your mailbox.
Check that two sessions of your account are not active - only one user per login is accepted
Try accessing again in a few minutes
If you are still unsuccessful accessing your account
Contact the Administrator firstname.lastname@example.org for assistance