The custom form could not be opened

Description: User cannot save items in calendar. The following error message shows:



"The custom form could not be opened. Outlook will use an
Outlook form instead. The form required to view this message cannot be
displayed. Contact your administrator."

Solution: Empty form cache in Outlook (Tools menu > Options > Other tab > Advanced button > Customs forms button (lower left corner) > Manage forms button > Clean cache button)