I am not sure if this Kopano or Outlook related - when a user adds a reminder to another users tasks, there is no reminder shown (they do set the reminder) unless a due date is also set to the task. The Task is added successfully and it also shows the reminder when editing, but never displays without due date.
It seems to work with or without due date on the users own tasks, so I think this might be Kopano related.
Server: Debian Jessie with up to date Kopano and OL extensions (Outlook 2013/Windows 10).
I have also tried to use the nightly OL extension, but it doesn’t change anything.