Can't set a featured image anymore

Hi. An issue happened to me today while uploading an article. I added a gallery and everything was ok. Then I wanted to add the Featured image, which was working perfectly just hours before (and there was no change in the website in that time period) and it didn’t work.

I checked and the Media Gallery works fine. The issue is with the Featured image option. When I enter there I can see this:

<img src=https://i.imgur.com/YuV4ME1.jpg>

I see than blank attachment that I can’t delete and there isn’t in the media folder (I checked by FTP) and I couldn’t find any “suspicious” file either.

The problem is since that blank attachment is there I can’t set a Featured Image, I can select it and confirm but it never appears in the right side of the post edition as it usually does once it’s selected.

On the other hand, I can still creating galleries from the Add Media button without any problem.

Tried deactivating all plugins and still doesn’t work.

Any idea what could it be?

Thanks!