Featured images now with 1px width/height

I saw that earlier there was this problem

http://wordpress.org/support/topic/wordpress-inserting-images-on-posts-width-1px-height-1px?replies=20 etc…

This came up at once, without any modification or work with the site (only posts are added).
Any featured image now has added attributes width=1 height=1 .

Of course, this can be easily solved with the help of CSS (!important), but I don’t like to WP dynamically generate wrong attribute values.
There’s no template modification, img metadata in database is valid, no plugin conflicts.

Tested with 2 functions for displaying images:
get_the_post_thumbnail
and
the_post_thumbnail

with various parameters (even forced width, height in parameter) but WP always add 1px for width/height.

Have no idea where he could be a problem, because I tried everything that was mentioned in the other topics.