Permalinks: Post name
Static Front Page? No
Polylang Settings:
-The language is set from content
-Hide URL language information for default language (Checked)
-Remove /language/ in pretty permalinks
-Detect browser language (Checked)
Main symptoms:
Block of notices thrown at top of page
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\frontend\choose-lang.php on line 65
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\frontend\choose-lang.php on line 52
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\frontend\choose-lang.php on line 53
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\frontend\frontend-filters.php on line 74
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\frontend\frontend-filters.php on line 74
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\frontend\choose-lang.php on line 163
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\frontend\choose-lang.php on line 171
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\include\links.php on line 148
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\include\links.php on line 148
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\frontend\frontend-filters.php on line 74
prefix="og: http://ogp.me/ns#">
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\frontend\frontend-filters.php on line 74
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\frontend\frontend-filters.php on line 74
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\include\links.php on line 148
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\include\links.php on line 148
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\frontend\frontend-filters.php on line 74
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\include\plugins-compat.php on line 116
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\include\plugins-compat.php on line 116
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\include\links.php on line 148
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\include\links.php on line 148
Notice: Trying to get property of non-object in \wp-content\plugins\polylang\frontend\frontend-filters.php on line 74Warning thrown by Events Manager plugin (if present)
Warning: array_key_exists(): The first argument should be either a string or an integer in \wp-content\plugins\events-manager\events-manager.php on line 410
wp_nav_menu falls back to wp_page_menu
wp_page_menu passes $args as an array, not object, so this also caused a string of notices from my custom Nav Walker
Now the tricky part is I honestly have no idea how this happens "in the wild." It was happening to client intermittently over the course of a couple months, always after they updated the site (I think the "intermittent" part is due to the W3TC plugin, but the issue still occurs with all plugins disabled) I'm as yet unable to duplicate the issue using their same steps; the only way I've been able to encounter it is by manually changing the value of the "pll_language" cookie to an empty string.
Unchecking "Detect browser language" does prevent the issue from happening.