A Google AdSense violation "Valuable Inventory: No content" for my homepage

Question

My site has Google AdSense working fine since 2015, but last month we got a violation for the homepage with these details:

Valuable Inventory: No content

As stated in our program policies, we may not show Google ads on pages with little to no value and/or excessive advertising until changes are made. This includes hosted ad pages or pages without content.

For more information, please review the following resources:

• Policy tips for creating high quality sites (part 1)
• Policy tips for creating high quality sites (part 2)
• Webmaster quality guidelines
• AdSense program policies

with this "How to fix" section:

My site's homepage appears differently based on the user:

• Anonymous users: will see login/registration form only - with no Ads
• Logged-in users: will see their homepage with feed from their friends - with Ads (in one place)

(And of course the login/registration form doesn't contain any code related to Google AdSense)

Actually, I noticed that Ads aren't fully restricted on the homepage, because sometimes they appear, but of course most of the time they don't.

Btn, I already tried the "REQUEST REVIEW" (without making any fix), but it didn't work.

Now, how can I fix this weird No content violation?!!