This applies ONLY to large thumbnails, not the ones represented by site icons, which seem to work okay (feel free to check the attached image to see how it looks). Tried to change the status of these tiles from just 'frequently used' to 'pinned' but that has no effect. This behavior is also consistent throughout Safe Mode and in case I remove the chrome folder from my profile. Interestingly enough, an extra profile I have as a backup with a copy of the old profile loaded the thumbnails for these same sites right away. Checked whether I have any differences in about:config settings concerning the New Tab between these profiles but have not found anything particular, same for the profile folders themselves. Can the creation of these thumbnails be controlled (e.g., can they be reset) somehow? UPDATE: One more thumbnail for a frequently visited site appeared today. Not sure why today or why just one. UPDATE 2: It disappeared along with the one in the top right. I do not understand why this is going on.