I get many void query in the mysql error log

This question may or may not be related to ISPConfig. My system is ISPConfig 3.1.2, Ubuntu 16.04, Mysql 5.7.18.
The mysql error log is flooded with these void queries. They happen about twice per 2 minutes. Are they related to ISPConfig 3.1.2? How do I find out which program or connection is responsible for them?

-------------------------
2017-05-01T17:27:15.908503Z 3 [Note] Access denied for user 'root'@'localhost' (using password: NO)
2017-05-01T17:29:17.320641Z 23 [Note] Access denied for user ''@'localhost' (using password: NO)
2017-05-01T17:29:17.321477Z 24 [Note] Access denied for user ''@'localhost' (using password: NO)
2017-05-01T17:31:17.404669Z 36 [Note] Access denied for user ''@'localhost' (using password: NO)
2017-05-01T17:31:17.405018Z 37 [Note] Access denied for user ''@'localhost' (using password: NO)
2017-05-01T17:33:17.482127Z 47 [Note] Access denied for user ''@'localhost' (using password: NO)
2017-05-01T17:33:17.482912Z 48 [Note] Access denied for user ''@'localhost' (using password: NO)

 

Which file? In the file /etc/crontab, there is no ispconfig entry:
---------
# m h dom mon dow user command
17 * * * * root cd / && run-parts --report /etc/cron.hourly
25 6 * * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6 * * 7 root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6 1 * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
#

 

I have commented out the 2 lines /usr/local/ispconfig/server/ in the root crontab and the non-sense queries persist.
It confirms that they are not relevant.

 

Yes, it is Monit and Mysql 5.7.18.
See the new connection method for new mysql:
https://mmonit.com/monit/changes/

if failed
port 3306
protocol mysql username myuser password mypassword
then alert