Redmine

Hi folks,

we have the issue on our Redmine Installation that we (not every user) get email notifications for tickets which are already closed / not assigned any more / don't have a due date any more.
This makes the ticket reminder notification useless.
Usually you get this Information only when a due date is set (1-, 3- and 7-days before), but now matter what, I get this notification permament.
It looks like, for some users, some mechanism got stuck who is updating this list.

I even don't get how this is triggered. The mails are sent every day 6o'clock, but I don't see any cron job. So I don't have any idea how to start an analysis.
We also did a Redmine Ticket (#22670), but so far no response yet...

Thank you so much for any help!
Best regards,
Markus

Installation overview (the issue existed already before we installed checklist/codebutton/Login_audit/Revision_branches plugin):

Environment:
Redmine version 3.1.1.stable
Ruby version 2.1.5-p273 (2014-11-13) [x86_64-linux-gnu]
Rails version 4.2.4
Environment production
Database adapter Mysql2
SCM:
Subversion 1.8.10
Git 2.1.4
Filesystem
Redmine plugins:
redmine_checklists 3.1.5
redmine_codebutton 0.1.0
redmine_contacts 4.0.3
redmine_contacts_helpdesk 3.0.1
redmine_login_audit 0.3.1
redmine_revision_branches 0.3.1