Are there known issues with shared calendars when the shared calendar
is not the "main" calendar, that replies to meeting requests don't get
registered?

One of our users reported it but I haven't seen it myself yet.
Could it be that Outlook only registers one calendar as THE calendar
for handling replies to meeting requests? And if this is the case, I
suppose there is not much that can be done about it? Or is there?

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Amedee