Hey folks, sorry for the late reply — unfortunately we are still investigating what could be the cause of these problems you're experiencing with your thumbnails and it seems that the only viable solution right now is to switch to the older version of our plugin, which you can find on the Developers tab of our plugin page: http://wordpress.org/plugins/wordpress-23-related-posts-plugin/developers/
We will keep you posted any new updates we might find along the way, but for the time being I'd like to ask you for your patience — we really do appreciate it! Thanks!
After all, if these problems persist or if you have any other thumbnail issues, do let us know what's the address of your blog, which version of the plugin you are/were using, and we'll gladly take a closer look at it. Hopefully after all this information we'll be able to get a hold of this problem sooner than later.
Again — thank you for your continuous patience and support! We're doing our best!
Take care & have a nice day!