Uploading/converting jpeg files with image doesn't work

When creating an image node uploading gif files works fine.

WHen I upload a jpeg file I get the following message:

* warning: imagecreatefromjpeg(): '/homepages/28/d41073845/htdocs/drupal/sites/www.tact.co-cornucopia.org/files/images/temp/bewerkt.JPG' is not a valid JPEG file in /homepages/28/d41073845/htdocs/drupal/includes/image.inc on line 288.
* Unable to create thumbnail image
* warning: imagecreatefromjpeg(): '/homepages/28/d41073845/htdocs/drupal/sites/www.tact.co-cornucopia.org/files/images/temp/bewerkt.JPG' is not a valid JPEG file in /homepages/28/d41073845/htdocs/drupal/includes/image.inc on line 288.
* Unable to create preview image
* The selected file /homepages/28/d41073845/htdocs/drupal/sites/www.tact.co-cornucopia.org/files could not be copied.
* The selected file /homepages/28/d41073845/htdocs/drupal/sites/www.tact.co-cornucopia.org/files could not be copied.
* warning: imagecreatefromjpeg(): '/homepages/28/d41073845/htdocs/drupal/sites/www.tact.co-cornucopia.org/files/images/marian bewerkt.JPG' is not a valid JPEG file in /homepages/28/d41073845/htdocs/drupal/includes/image.inc on line 288.
* Unable to create img_assist_properties image

The jpeg file itself is uploaded though and presnet in the /images and /images/temp folder.

What can I do about it?

Looking forwards to hearing from you.

Kind regards,

JanPieter