I noticed search.google.com complaining about an invalid date format in the generated sitemaps. After closer examination, I found, for example:
2023-04-26T08:55:49+0000

However, Google wants:
2023-04-26T08:55:49+00:00

Thanks, already known: https://github.com/zenphoto/zenphoto/issues/1369

Fixed in 1.6.1a

Thanks