Failed notifications in CEE 1.5.0p24

I have updated from CEE 1.5.0p16 to 1.5.0p24 recently. I noticed that email notifications started to fail quite often since the update. Looks like only bulk notifications fail (not delivered to users). Bulk notifications are configured just outside of working hours.
Other notifications seem to work quite well.

Unfortunately I can’t see the reason for failing. I have checked:

and didn’t find anything related like errors or crashes or failures.

Also I have checked the connection between the mail server and the MTA and it works good.
As I could see the notifications are collected in bulks properly and the system tries to process sending them out after 8:00, but some of bulks were never passed to MTA.

Does anyone have an idea where to look?

Best regards
Hermann M.

That means you see inside the log of your monitoring system that the bulk is getting processed and in your local MTA (postfix/sendmail/nullmailer) log you see nothing?

right, this is what I mean.
I am struggling to find the place, where to dig for details.

Kind regards
Hermann M.

Found in var/log/notify.log a relevant message:

2020-06-29 08:01:07 Notification plugin did not finish within 60 seconds. Terminating.
2020-06-29 08:01:07 ERROR: script /omd/sites/mastersite/share/check_mk/notifications/mail --bulk returned with exit code 1

Nothing in /var/log/maillog to this item.
Any ideas?

Kind regards
Hermann M.