Thank you for your post.
For those fields formatted as Drop-down calendar, did you also select the sub-option "Include icon to show and hide calendar"?
What other applications are open when the drop-down calendars stop functioning?
Do you recall what you were doing just prior? For instance, had you jumped to another application and then back again?
Any other information you can provide may be helpful in narrowing down the cause.
At the time that your calendar drop-downs stop working, I'd be curious to know if other (textual) dropdown lists have also stopped functioning. I have seen a number of times (not seemingly related to the memory leaks) where all drop-downs go dead, including those that are part of FM itself (for example, the layout list won't drop down in any mode; so the only way to manually switch layouts is to use the book to go one-by-one). I haven't previously reported this because I haven't been able to replicate it on-demand...but it does happen to me every few days. Note however that mine is on Windows 7, whereas it looks like you are on OSX.
I would have thought I had replied to this thread a couple of times since it appears the last posting was made...how time flies. But, since I have started using v15 (only a few months) this issue seems to have stopped happening.
However, I have run into another issue in this same file in the past week or two - body records get blanked out. You scroll them on/off the visible screen and they sometimes show up as they should, but other times just show as empty gray rows. I'm working on some documentation and screenshots for a separate thread, though.