Sidebar missing pages

[Ulrich Kaiser]

The tree structure is the following 

PAGES
-Main
--Content1
--Content2
-Problem
--Content3
--Content4
-Solution
--Content5
--Content6

Now, one day the space homepage somehow disappeared (probably users fault). 
Via edit space details, I set the homepage to Main. 
Then, if I open confluence, it only shows 
PAGES
-Content1
-Content2

Nothing else... 

If I go via recently modified pages, open Content3 page, the side bar gets updated to:
PAGES
-🏠Main
--Content1
--Content2
-Problem
--Content3
--Content4
-Solution
--Content5
--Content6

[Ulrich Kaiser]

Does anyone have a hint how I can see my old tree again? Any idea?
I tried to clear the space homepage field. But it would not let me save with a blank field. 

If I change the sidebar to show all the pages in an Title - A to Z structure, it shows the missing pages....