[Plugin: NextGEN Gallery] Thumbnails distorted

My photos are among my folders in the correct size and dimension. In the gallery in the admin area but they appear distorted (in the wrong dimension). The same occurs in the public gallery.
Please, see: new example
This error occurs only for some time on new galleries, but not in older galleries. Please, see: old example
My NextGen Gallerx version is V1.5.5
What is the reason? Thanks for help.

http://wordpress.org/extend/plugins/nextgen-gallery/