@Damjanovska, what version of RW did you upgrade from? Are you using Stacks, if yes, what version? What version of OSX?
Did you follow the upgrade steps as outlined here
As for the sidebar, can you move your mouse to the left edge of RW and see a bar & arrow? If yes, click and drag the pane open.
As for thumbnails, they are an issue in 6.3.5, you can download the 6.3.6 GM beta, please note in RW Preferences there is now an option to save thumbnail of site.
As for importing pages from different project files, it sounds like you have some data corruption going on, perhaps do a restart and verify your HDD. This sort of issue is not one that I have seen reported prior, so it likely is an issue with your machine.
Lastly, there were some issues with 6.3.4 that this fixed
Brad