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
- there was a file named "BookmarksExtra" (with no extension) that i simply renamed
- 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!!!