This message will occur if the token used for
DayHeader minicalendar options converts to a blank space (ie. its translation is a blank, or it does not have a translation) and you asked for the 1st character of the translation to be uppercase.
You should either select another token to be used for DayHeaders, or make sure that a translation is available for all weekdays.
Topic 175860, last updated on 19-Apr-2020