My zimbra server version is ZCS 7.1.2
I got this error with 1 email account only. rest of all email id are OK.
I just removed the external account and recreated again.
Still the same error.
I just enabled debug using "zmprov addAccountLogger
user@domain.com zimbra.soap debug" command.
In which Log file i can see the complete error details which is related to external datasource.