Support opening a calendar entry WITHOUT the calendar view context, for now it is not possible.
The dialog box suggest that selecting should work but it was not intended to do so. The complete URL which is working is the whole one! Calendar entries using iNotes or Verse have to be opened in context of the calendar view and not without this is the limitation right now.
Currently there is no other way, then using the unmodified URL from the window. As mentioned context information is missing in the browsers object model, which is creating the error message. Right now, please use the unmodified URL like ( https://www.mysite.mycountry/mail/myfile.nsf/($Calendar)/97CFB086FD3F5CC9C1258527004428E7/?OpenDocument&PresetFields=s_ParentContentId;($Calendar)6,s_ViewName;(%24Calendar),s_InheritAttachment;false,ThisStartDate;20200312T130000Z,ThisEndDate;20200312T130000Z )