I faced this issue once and tried below mentioned workaround. You can give it a shot:-
Work Around
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1) zmprov ga
user1@domain.com | grep zimbraId
2) mysql -e "update zimbra.mailbox set contact_count='NULL' where
account_id='c1476c9e-47fe-3c42-984a-3d834b3aa9s3'"
3) Restart zimbra