Not sure if this is the same as what was seen in the earlier post, but I've seen this where you can't start entering data until locking the index and selecting a page.
To recreate with a new reference manual in a new (5E) campaign:
- Open Builder with no data
- Hit New Chapter, the view now looks like this: Capture1.JPG
- Click on the <<New Page>>, the right hand panel is blank with no options or ability to enter text
- Add a few pages and give them names, the right hand panel remains blank even though it looks like Page 2 is selected: Capture2.JPG
This seems to be because you cannot select a page while the index is unlocked for editing. It selects the page name for editing, but doesn't change the actual page view - perhaps it could do both in the unlocked case?
Once you lock the index, you can then select a page and begin editing as expected: Capture3.JPG