Calendar errors: The value input for field Start date is invalid

When viewing a calendar (project, own or all) at first there are entries corresponding to task dates in current month. When clicking 'prev' or 'next' arrows AJAXed part of the page refreshes, month changes but this month entries can be seen only for a second, then they disappear. After that, calendar is empty no matter what month or view mode (week/day) I choose. After full page refresh (F5) current month with its entries is back and messagebox with the following errors shown:

The value input for field Start date is invalid:
The value 2014-02-23 does not match the expected format.
The value input for field End date is invalid:
The value 2014-04-06 does not match the expected format.
etc.