Thumbnails and resizes broken

Since 2004 I've used Gallery 2. It worked perfectly all that time, until some months ago. Resizes weren't created properly anymore when I uploaded new photos, and the gallery displayed the 'missing-image' symbol where these resizes should appear. Because of this I decided to upgrade from Gallery 2.3.1 to Gallery 3 and import all 12195 photos there.

Unfortunately, the problem persisted.
So I tried updating the images, and repairing the gallery from the maintenance tab.
I tried varying the sizes of the thumbnails and resizes in the advanced tab and in the theme options tab.
I tried rebuilding the thumbnails and resizes with the 'Rebuilt items' plugin. (This plugin did nothing, literally)
I deleted the var/thumbs and var/resizes directories, and created new ones (777) and copied the content of var/albums to them, and then tried to resize these files with the update images tool from the maintenance tab. This replaced most of the files I just copied to var/thumbs and var/resizes into images of a broken image (see the attached file).
Others were left unchanged and the rest is turned into 200x200 images (thumbs and resizes both! Even though the preferred size for a thumbnail is set to 150 and resize to 800).

I have no clue how to let the Gallery create proper thumbnails and resizes at this point. Hoping one of you guys might help me out.

Thanks, Rolf

P.S. The gallery is located at http://jvsolagratia.bwkrimpen.nl/fotogallerij/

ImageMagick was selected as active toolkit. I've changed it and I'm using GD now.
It's way slower than IM but it seems to do the trick!

Thanks for your help!
Rolf

Will do!
Thanks again