Hi,
I would rather do Remote Session about this to show as not that easy to write every piece of information I am experiencing (quite long time GW 2014+) with shared calendar items, I tried if anything happened after upgrade to GW18 but its still the same. Guess I will try:

Lets say I have 2 GW users Assistant and Boss.
Boss created shared calendar item and shared for full access to Assistant.

1.) When Assistant creates appointment in bosses shared calendar, only place he can change anything about it is Sent folder, not able to Edit appointment in calendar view (totally wrond place to be when asistant have tons of items in Sent folder to find just one appointment.

2.) When Assistant find and appointment in Sent folder, he can Edit it (only able to see Edit option when I click in "To" field of item, nowhere else) . So if he edits lets say time of appointment, new appointment is created. Same one with previous time stands there. Sometimes when I open old appointment, it shows me error d107. Sometimes not.

3.) Eventho assistant creates appointment in bosses shared calendar and eventho hes not able to Accept appointment afterwards (as he created it), there is still in Properties Undelivered:1 Recipients: 1 (Assistant himselve)

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Stranges thing is, when boss creates appointment in his shared calendar it acts different way.

Thanks for any help.
Lukas