Not a valid template error since updating to 2.040

@lgoo – There could be a conflict with another plugin that is causing this issue, or the template you were using is no longer available due to the way WordPress updates plugins and how the folder / files are affected.

If you need help rolling back to a previous version this article will be helpful: http://www.nextgen-gallery.com/how-to-rollback-a-version/

Thanks!

– Cais.