New document page in Illustrator opens incomplete.

Hi. Have you tried resetting the preferences? https://helpx.adobe.com/illustrator/user-guide.html/no/illustrator/using/setting-preferences.ug.html

Without any system info and possibly other technical details nobody can tel lyou much. This simply looks liek an issue with hardware accelerated UI drawing functions. If you're on a windows machine, updating your graphics driver would be the first thing to do.

Mylenium