I wrote email to booken support then.
Tanks for quick response

.
Meanwhile i think i've found solution to my problem on this site:
https://wiki.mobileread.com/wiki/Cybo...roubleshooting
They wrote:
1. If the unit had been re-displaying the second startup screen for sometime before it was reset or if you are unsure how much power remains, connect the USB cable and allow time for a full recharge before continuing with the next step.
2. Re-flash with version 796fmt [1]. Any books that were stored on the internal memory will be deleted. Version 796fmt fixes the problem because it fully reformats the internal flash whereas version 796 does not.
I will try this method.
Edited
Yey it worked

I think it is good idea to remowe all files from inernal memory before upgrade to version 2.0/1.5. I think I had some corrupted files.