I have found a bug, and have a guess as to why it sometimes occurs.
It seems, when you are displaying several months and have set a max low date, the month drop down can break. You can't change the month, until you change the year or press prev/next month. This happens when the month drop down, is located on a month with a different year than the current.
Mt guess is, when the month drop down selector is located in a different year than the current day, the default year is displayed correct but is in reality the current year, thus making the month selections invalid by the max date. However when you yourself pick that year, it is then treated corrects thus enabling month selections.
To reproduce the bug, just set the maxDate at '1d' and numberofMonths to 1 + the month number (jan = 1).