Not sure if anyone has run into this but could not find it on answers and would be helpful to have on here for future searches.

You only get a warning if a required field is omitted.

However I don't believe it actually creates the file on the disk, because the next activity which tries to attach the file created from the activity

Throws this misleading error:

So after spending a lot of time trying to understand why programs would not have access to ProgramData (which is very unexpected since this is THE place for programs to access the disk), we decided to handle the warning and fill in the EndDate.

Sure enough this solved the "Access to the Path ...\ServerData is denied error".

My guess is that the iCalendar files are not being created if any fields are omitted and the attach electronic calendar is assuming, since it can not find a file, that it has simply been denied access.

If the iCalendar file threw an error saying no file created, that would prevent the workflow from moving forward.

This is in Workflow 10.4.1.223