Moved server, and WP now uses original images for all thumb sizes?

Question

I just moved servers.

When calling wp_get_attachment_image() or any of its sister functions the image returned is always the full-size image, scaled down, instead of the requested thumbnail size. This has a pretty nasty impact on load times.

Both AJAX thumbnail rebuild and Regenerate Thumbnails those have happily created plenty of thumbnails. WordPress just chooses to ignore them.

Thanks for any help. This might be a dupe of https://wordpress.stackexchange.com/questions/38987/major-issue-with-image-sizes-being-lost-only-full-size-images-being-shown but I'm not sure.

Update

After some research, it appears that the attachment_meta => sizes array on each image has all the correct keys for sizes, but none of the values:

(
[width] => 677
[height] => 472
[hwstring_small] => height='89' width='128'
[file] => my_original_image.jpg
[sizes] => Array
    (
        [thumbnail] => 
        [medium] => 
        [large] => 
        [admin-list-thumb] => 
        [select-a-post] => 
        [detail] => 
    )

Why might this be happening?

Answer 1

I'd be sure that the new server has the same php config as the last, mainly GD library or ImageMagick. Use phpinfo http://php.net/manual/en/function.phpinfo.php to check configs if it's shared and you can't get to the php.ini file.

Answer 2

Found the problem.

It turns out somebody had done a slightly rough search and replace on the DB, breaking the delicately serialized data in wp_postmeta. thankfully I had a backup.