dovecot - May 2017 - dovecot log shows: service=imap ... Error: Temp file, creation to /tmp/dovecot.imap. failed: No such directory

I am using dovecot 2.2 with plesk 12 and I have made no modifications to 
the dovecot config.
>> Logwatch reports the messages below.
>> The errors don't seem to appear for every imap connect as can be
seen from the number of deliveries and successful logins.
>> So what makes dovecot fail sometimes to create the /tmp/dovecot.imap?
>>
>>
>> Dovecot Deliveries: 19
>>   Dovecot IMAP and POP3 Successful Logins: 106
>>   Dovecot disconnects: 72
>>
>>   **Unmatched Entries**
>>      dovecot: message repeated 4 times: [ service=imap, user=tester at
example.com, ip=[::1]. Error: Temp file creation to /tmp/dovecot.imap. failed:
No such file or directory]: 1 Time(s)
>>      dovecot: message repeated 5 times: [ service=imap, user=tester at
example.com, ip=[::1]. Error: Temp file creation to /tmp/dovecot.imap. failed:
No such file or directory]: 1 Time(s)
>>      dovecot: service=imap, user=tester at example.com, ip=[::1].
Error: Temp file creation to /tmp/dovecot.imap. failed: No such file or
directory: 10 Time(s)
>>      dovecot: service=imap, user=tester at example.com, ip=[::1].
Error: Temp file creation to /tmp/dovecot.imap. failed: No such file or
directory: 1 Time(s)
 >> Maybe you are chrooting dovecot yourself? The chroot must contain 
/tmp directory. <<

If this was the case I would expect the error for every imap connection 
and not only sometimes.
Nonetheless: How can I check the chroot settings?
Peter