Images disappearing from gallery block

[arnopeck]

After uploadingg imagges in a gallery block, when i save some images are missing, from the gallery.
Even checking the media gallery some images are missing.
When i come back to the editing page i can see JS error about wrong JSON.

To Reproduce
Steps to reproduce the behavior:

1. Edit an article or a page
2. Click on 'insert Gallery Block
3. Drag images (or click select images)
4. Save Article
   5 See the problem

Expected behavior
Just have my gallery on the article or page.

[Soean]

What is you user role? We have an open issue, if you are not an admin. See #9875
Should be fixed in 3.9.

[arnopeck]

My user is administrator...

[bahia0019]

Does it act similar to this? #6861

[arnopeck]

@bahia0019 Not really, i don't see flickering or disappearing images like in #6861,
when i upload the images i can see the preview and it looks fine, then i save, and see the frontend,
but when i come back in admin (reloading the page) images are missing...

Some times gutemberg is asking me to reset as block the gallery, but when i click one one of the 2 buttons available it just insert on the text block a UL LI list (with invisible elements)

[designsimply]

When i come back to the editing page i can see JS error about wrong JSON.

Does the error message that you are seeing say, "The response is not a valid JSON response"?

Is there any chance you would be able to check the Network panel in your browser's web developer tools at the time the error appears to see if there is any more detailed information about the error? Here is an example of a successful response in a test I did, and we would want to know if you see a 400 or 500 error instead of a 200 response:

If you are able to check with a plugin, noting the details from the Server section in Dashboard > Health Check > Debug screen found in the Health Check plugin as well as noting what size the images are that you are trying to upload would be good things to document if possible.

[youknowriad]

Looks like a duplicate of #8935