If you are only using 341mb, then you may be over you inodes quota. This can be seen in you cPanel Stats panel. An inode is any "thing" on the file system (file, folder, symlink, etc.). Each email is a file.
Is crdncycl@ the user you are trying to access in webmail? If so, check Email Accounts in cPanel for Usage and Quota.
Can you create a new file in File Manager? If not, it's probably inodes.