@azzcat: a few comments ago, I posted a download link to WordPress Popular Posts 2.1.1 Beta. Please install it, it should fix the title issue you just reported.
@wramos: until now, nobody had ever reported another plugin conflicting with WordPress Popular Posts. You're already doing a lot to help me out, so thank you for that :) Anyways, please do deactivate them all, and then proceed to enable WPP alone. If everything works as expected, then go and activate HeadSpace2 and check again. That'll tell if you if HS2 is the one causing the issues.
By the way, I did some research and found that the get_the_post_thumbnail function (used by my plugin to retrieve thumbnails selected by the user) appears to be a bit buggy:
Reported by: Denis-de-Bernardy
it seems like the new post thumbnail feature neither crops images, nor uses thumbnail images when available.
get_the_post_thumbnail() and set_post_thumbnail_size() seem to use the size of post-thumbnail, but image_downsize() only seems to understand a size of thumbnail.
this results in huge images in posts, even when the size is small.
The guy who reported it also posted a patch that might solve the issue.
This might explain why WPP won't use the image you selected and displays the default thumbnail instead.
I'm going to wait for a bit until a new version of WordPress is released hoping that it fixes the no thumbnail issue you reported earlier. If it doesn't, then I'll open a new ticket on WordPress Trac to notify this issue again.