Frequentl failures to create /tmp/dovecot.imap

[Peter]

I am using dovecot 2.2.27 with plesk 12 and I have made no modifications 
to the dovecot config.

my log file frequently (but not always!) reports errors that 
/tmp/dovecot.imap could not be created.
The errors don't appear for every imap connect but for about 5% to 10% 
of the time.

I already attempted to solve the problem by adding the following line to 
/etc/dovecot/dovecot.conf but that did not remove the error:
  mail_temp_dir = /tmp

So why does dovecot mostly work without errors but sometimes fails to 
create the /tmp/dovecot.imap?

..excerpt from /var/log/syslog...
     dovecot: service=imap, user=user1 at mydomain1.com, ip=[::1]. Error: 
Temp file creation to /tmp/dovecot.imap. failed: No such file or 
directory]: 1 Time(s)
     dovecot: service=imap, user=users2 at mydomain2.com, ip=[::1]. Error: 
Temp file creation to /tmp/dovecot.imap. failed: No such file or 
directory: 10 Time(s)


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