Chouby on "[Plugin: Polylang] Fatal error once logged out"

You should consider asking to your theme support as the error is in theme.

The problem is that the theme believes that WPML is active when it is not. Polylang does provide documented function of WPML but does not provide internal functions such as get_active_languages().

So either the theme author can support Polylang or he has to make a better test to detect only WPML.