@MarkRH: You need to enable the cacheManager plugin to get button to clear all caches manually.
@mondeduvelo: In the future please switch Zenphoto to the native English to post error messages (or at least translate them beforehand). We sadly don't speak French and it saves us time to help. Thanks.
It seems the sitemaps generated misses a doctype define like
<?xml version="1.0" encoding="UTF-8"?> at the beginning. We will look into that. However according to sitemap validators it is considered just a warning and nevertheless considered valid XML. Therefore it should work.