Polylang not detecting browser language

Hi, I’ve been using Polylang (free) for more than a month on this website translated into 4 languages. Polylang is set to detect browser language, and has a default language EN.
After some other plugin updates/(de)activations, the website always defaults to English language, even when the website was visited before in another language: Chrome dev tools shows that pll_language cookie is set to – for example – NL. Even then, a next (non-incognito) visit will default back to EN language. It seems as if the cookie is ignored (?).
Even stranger: even when I’ve deactivated Polylang’s language detection and switched Polylang default language to NL, the home page will still be displayed in EN.

Things that I’ve tried further:
Delete browser caches (Safari and Chrome)
Work in private/incognito browser modes (Safari and Chrome)
Deactivate caching plugins (Autoptimize and WP Supercache)
Deactivate all plugins – Done, except for Polylang and SiteOrigin Widgets Bundle. Deactivating either one of these blocks access to admin area and forces me to restore a backup of WordPress database to regain access. I have no way (that I know of) to deactivate or delete these 2 plugins.
Switch theme to Twentyseventeen (with only above 2 plugins active) – of course the site and menu is basically broken, but the issue of defaulting to English remains in place.

My settings:
WordPress and plugins all up-to-date
Theme: LAVA by Themespirit https://www.themespirit.com uses SiteOrigin Page Builder and its templates plus widgets
Permalink structure: https://www.lemoulindelande.com/%category%/%postname%/
Static front page called “Home” and translated into the 4 languages
Polylang URL modifications settings:
– language is set from the directory in pretty permalinks
– remove /language/ in pretty permalinks
– front page url contains language code instead of page name or page id

Any suggestions? Help is much appreciated!

The page I need help with: [log in to see the link]