Well, finally i fixed it. Yes, Clive was absolutely right - **default value MUST be set ONLY** this way #default_value' => **date('Y-m-d', $date) not depending #date_format value for date_popup field.**
Difference was that I used 'Y-M-d'(same as in #date_format) instead of 'Y-m-d' for setting #default_value.
Thanks everyone. Hope this helps someone.