Accepting participant in roundcube calendar

[guldendraak at gmx.net]

Hi List

I am experiencing a problem with invitations between the roundcube calendar and outlook. When creating an event in roundcube with a participant, that participant is sent the invitation correctly. Upon accepting the invitation, a corresponding message from the participant is received in back roundcube.
In roundcube, that message is recognized as acceptance and a button "accept participant" is displayed. However, when I click on this button, an error message is displayed: "A newer version of this event already exists! Aborted." This is reproducible, i.e. happens every time. Any hints what the problem could be?
Thanx!