Fatal error slide.php

I manage the site https://www.sportpeople.net, built with Xanthos Theme. The site worked well but, two days ago, without any change to our site, we had this fatal error log on the public home page:

Fatal error: Call to undefined function xan_slide_image() in /web/htdocs/www.sportpeople.net/home/wp-content/themes/Xanthos/slide.php on line 17

We tried a deactivation of all the plugins but without results. We took the slide.php from a backup version and it was good. But, unfortunately, after some hours, the problem went back. Do you have a solution?
Thanks a lot