Hello, Please follow my step. Maybe it can be useful for you.
1) Enter DB of the shop.
2) Find the table ps_cms_category. Enter it.
3) First take backup for this table. Then change id_parent to 0 for all records.
4) Then again try to navigate CMS pages.
Also enable HTML option in BO (under General tab).
Let me know about the result.
If this way will be useless restore the modified table, and share image of problem here.