• Alice delegates a calender and a mailfolder to Bob
• Alice creates a mailfilter which copys all mails from resources to the mailfolder
• Bob subscribes to the calender and the mailfolder
• Bob creates and identity (alice) in his account
• Bob creates an event in Alice' calender und sets the alice id as organizer
• Bob invites a resource to this event
• Bob can see the answer from the resource in the mailfolder
• Bob sees in the answer "The event is not found in your calender"

How can Bob record the answer in Alice calendar?

I confirm that this is indeed an issue. When handling itip replies Roundcube
looks in personal (and writeable) folders of the current user. In this case
the event is in a folder of the delagator. A fix would be to additionally
check available/subscribed calendar folders of all user delegators.