Test case:
Open the statistics window. Press the "first" button. (calendar jumps to january
13th, which is apparently the first day on which I have data). Next, press the
"next" button.

Expected result:
Calendar moves to the next date for which there is data, i.e. january 14h

Actual result:
Nothing happens

Alternative scenario:
Click manually on january 14th, then press next. (yields same result)

Alternative scenario:
Click manually on any other date, then press next. (works correctly)

Alternative scenario:
Click manually on any date, and pres "previous". (works correctly)

For me, this reproduces 100%.

You probably have a duplicate date in your saved history.... Should be ok now.