MOCCACAL-12 / MOCCACAL-68 is back, at least for "subcalendars".
Description from http://xwiki.475771.n2.nabble.com/Mocca-calendar-display-short-description-instead-of-title-in-month-week-day-view-td7597592.html#a7597610
Short version: creating two events with the same title does not work for new Calendars (i.e. every calendar but the preinstalled "Other Events")
The problem is that the check to create new unique page names happens in the base space "MoccaCalendar", while the event is created in the space defined by the new calendar.
The situation is made somewhat more urgent now that new calendars define a space for themselves but also has been present before nested spaces;
moving a calendar to a separate space would e.g. cause this issue, too.