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The message was shown when all the settings were unselected, making the plugin inactive and generating this error message.
It seems that a fresh configuration fixed it all 🙂
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btw, other message that I saw were :
Warning: foreach() argument must be of type array|object, null given in /wp-content/plugins/wp-admin-cache/index.php on line 69
Warning: Trying to access array offset on value of type null in /wp-content/plugins/wp-admin-cache/index.php on line 192