1.7 and Flex Pages: PageLegacyTrait::move(): Not Implemented error

Hello!

I made a clean Grav install from 1.7 branch (rc7 tag included) and used my own existing pages /user/pages.
Also - changed page type (PLUGIN_ADMIN.PAGES_TYPE) to PLUGIN_ADMIN.FLEX.
Now pages themselves are showing OK, but when editing and saving page, following error is
shown:

Grav\Framework\Flex\Pages\Traits\PageLegacyTrait::move(): Not Implemented

Any helping instrucions, please?

Regards,
Raivo