>> It sounds like your NFS is somehow managing to break the files, this
can happen easily if same user is accessed by two backends concurrently.
>
> how it's possible to resolve?
By not allowing the same user to access two backends concurrently.
One option and the best way is using dovecot director. However, it is soon to be
removed from free community dovecot.
Another way is using a proxy (example HaProxy) with sticky IP. The disadvantage
of sticky IP is if they are using multiple IP's such as a desktop and mobile
connection at the same time.