Ok, there’s really only one reason that happens. There’s an image so large that it causes PHP to run out of memory when Connections tries to create a cached thumbnail of the image. Previous version of Connections used a third party library to do image scaling “outside” of WordPress. Now Connections uses WordPress functions, these come with a bit more overhead which is why it worked before and does not now. I help prevent this from happening… on the initial upload Connections will scaled large images down so when thumbs need to be created it will not be an issue (hopefully).
The solution, log into your server with FTP download the images for entries that being with “T” found in this path:
Resize any large images to be smaller and upload them overwriting the large versions on the server. Important! Keep the same name.
My experience with the couple others who ran into this it was only an image or two that needed fixed.
Hope that helps!