Since a few weeks, i can't customize the thumbnail any more when i add a new page in my favorites. I do it but when i open Speed Dial, the link appears as a classic thumbnail with a random color background and the (white, most of the time) url in the foreground.
Worst, if i modify an old link that was already customised before this bug appears, the thumbnail goes back to a classic text-over-colorfull-background item…
It's sad because it was a really great way to quickly find items in Speed Dial!

Up to date Opera for Windows (v54.0.2952.54 as i am writing)

YES!!! I renamed "BookmarksExtra" and Speed Dial has gone back to normal operation!
Thank you very much for your help @zalex108!

For those who are also in trouble, here is exactly what i've done (Windows 7 64-bit, Opera for Windows v54.0.2952.64 Stable) :

• opened the "O" menu and clicked on "About Opera" ;
• under "Paths" i noticed [Ctrl+C] the directory path where my profile is stored ("C:\Users\MYNAME\AppData\Roaming\Opera Software\Opera Stable") ;
• quit Opera ;
• opened Windows File Explorer, pasted [Ctrl+V] the directory path in the address bar and hit [Enter] ;
• there was a file named "BookmarksExtra" (with no extension) that i simply renamed [F2] "BookmarksExtra2" ;
• reopened Opera, opened a new tab and… "Oh no! more lemmings! all my custom thumbnails disappeared!!!" "Don't panic. It's OK." ;
• quit Opera again and noticed it created a new "BookmarksExtra" file ;
• deleted this new file and renamed [F2] the old "BookmarksExtra2" back in to "BookmarksExtra" ;
• reopened Opera and… tada! my custom thumbnails reappearred, the one i loose during the bug period are customisable as before and i can add new ones!
• let's say it again: YES!!!

YES!!! I renamed "BookmarksExtra" and Speed Dial has gone back to normal operation!
Thank you very much for your help @zalex108!

For those who are also in trouble, here is exactly what i've done (Windows 7 64-bit, Opera for Windows v54.0.2952.64 Stable) :

• opened the "O" menu and clicked on "About Opera" ;
• under "Paths" i noticed [Ctrl+C] the directory path where my profile is stored ("C:\Users\MYNAME\AppData\Roaming\Opera Software\Opera Stable") ;
• quit Opera ;
• opened Windows File Explorer, pasted [Ctrl+V] the directory path in the address bar and hit [Enter] ;
• there was a file named "BookmarksExtra" (with no extension) that i simply renamed [F2] "BookmarksExtra2" ;
• reopened Opera, opened a new tab and… "Oh no! more lemmings! all my custom thumbnails disappeared!!!" "Don't panic. It's OK." ;
• quit Opera again and noticed it created a new "BookmarksExtra" file ;
• deleted this new file and renamed [F2] the old "BookmarksExtra2" back in to "BookmarksExtra" ;
• reopened Opera and… tada! my custom thumbnails reappearred, the one i loose during the bug period are customisable as before and i can add new ones!
• let's say it again: YES!!!

I also really dislike this feature, it is truly exasperating! As previous posters, i prefer going back to where i was when leaving Opera to continue my reading.
I deeply believe that developpers (from Opera or every other software) should never introduce that kind of feature as mandatory (aka the Google way of life: we decide what is good for you): user must have the ultimate choice. Such a feature have to be an option, defined by the user in the settings.
In my own opinion, i agree with @booopera when he says that it is easy to open a new tab! Opera for Android has a great interface and i don't understand why some people asked for this feature… It is so easy to open a new tab when YOU choose to do so!
Please, Please, Please make it an option!

Since a few weeks, i can't customize the thumbnail any more when i add a new page in my favorites. I do it but when i open Speed Dial, the link appears as a classic thumbnail with a random color background and the (white, most of the time) url in the foreground.
Worst, if i modify an old link that was already customised before this bug appears, the thumbnail goes back to a classic text-over-colorfull-background item…
It's sad because it was a really great way to quickly find items in Speed Dial!

Up to date Opera for Windows (v54.0.2952.54 as i am writing)