It's already the third day when plugin shows me error instead of tip:
SELECT * FROM wp_dailytipdata WHERE (Display_Date='0000-00-00' AND Display_Day = 0 AND Shown_Date='0000-00-00' AND group_name='Tip');
I had tips perfectly planned (it wasn't because I forgot to add tip). I checked all dates of planned tips and everything was OK until next day, when tip, which was supposed to show, was set to previous day. (For example, I set the date 2013-01-25, but today the tip has the date 2013-01-24). It looks like the plugin changed the date and that also caused this error message.
When I updated the tip, everything was OK and will be until tomorrow, when error will show again. But I noticed, that today's tip's date changed again to 2014-01-25.