Some Image Styles not created

Hello
I hope I do not bother you with my hopefully stupid issue. I updated in the last days the colorbox module, but i don't think it's a colorbox issue.

Last week after a plupload, the storage of all data for the images in the database and the creation of all image styles did work fine.

But now just the thumbnail styles images are created. Even in a colorbox slideshow the image style large would not be created.

I checked the .htaccess-File and the permissions. All OK (thumbnails are created so far).
I also checked the Config of Styles, Media and Colorbox. All seems to be OK. I didn't change the Settings before.
Any Idea? Thank you for your attention.
Hans