WPML can modify the return value of get_home_url() to include the current language slug (e.g. http://www.domain.com/en/). In this case, the base URLs used by lightbox-photoswipe to determine if the file is local or not do not work, since the path to the image does not include the language slug. The solution proposed my WPML developers to get the site URL without the slug is to use get_option('home') (https://wpml.org/forums/topic/get_optionsiteurl-returns-the-language-site-url-not-the-wp-url/).
Appears to be this commit which created the problem: https://github.com/arnowelzel/lightbox-photoswipe/commit/01d1d71b8b391dbf71a334f1f991332d14096b3a
