Thumbnail image not used although generated

Hello everybody,

I have the almost same problem as ARubinstein had six years ago with WordPress 2.5 in http://ww.wp.xz.cn/support/topic/thumbnail-image-not-used-when-inserting-image-in-post but now with WP 3.9.1.
I use the_post_thumbnail() in my theme, with a parameter size “xsmall” to output a thumbnail.
The output
<img width="120" height="93" src="http://xxx.de/wordpress/wp-content/uploads/My-Pic.jpg" class="attachment-category-thumb wp-post-image" alt="My Pic">
As you see, the size is set correctly, but although it exists and was generated by WordPress, the image http://xxx.de/wordpress/wp-content/uploads/My-Pic-120x93.jpg is not used.

It does not matter wether the thumbnail was generated in the first place or using the “AJAX Thumbnail Rebuild” plugin. Also, the problem does not occur for every image and can be resolved if the image is saved anew (export from jpeg to jpeg).
However, for a non-expert, it is not easy to spot, while it increases loading times big time (full image is ~ 5MB).

Anybody any ideas? Maybe just a straw, like e.g. some information about how WordPress stores the information about which thumbnails lie where (or could I just manually create thumbnails and WordPress would use them, i.e. it checks for file existence?)

Any help appreciated, thank you!