[Plugin: NextGEN Gallery] Problem uploading images WP 2.6.1 and NGG 0.98

I am experiencing difficulties uploading images using wp 2.6.1 and ngg 0.98.

To upload images, I create a zip file and upload them into a gallery.

But when I try to upload, wordpress simply returns a blank page.

When I check the directory on the server, some times (a) the images have been uploaded and saved to the disk, but are no listed in the Gallery; at other times (b) the images have not been uploaded to the server.