[Plugin: NextGEN Gallery] Plugin crashes on "create new thumbnails"

I have tried several times to upload some images but cannot get past the create thumbnails stage.

First attempt I added the files by FTP, then clicked “scan folder for new images”. The progress bar gets to 100%, but then the dialogue box remains on the screen, with the title “Create new thumbnails” and no other text or images except an X to close. If I click the X, the images have been imported into the gallery, but no thumbnails. I’ve tried in Chrome and Firefox with the same result.

Next I tried to upload the images as a zipfile. The same behaviour with the dialogue box occurred. There are 10 images in the zipfile. The first image got a thumbnail created however, but none of the others.

Is there any reason why images might fail on thumbnail creation?

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