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You don't have permission to access /forum/admin.php on this server.
I get this error while trying to search a for a phrase on admin. Only happen why typed on "text contains" and with the phrase "You must log in or register to reply here"
I've been getting this error for a while now, even on Xenforo 1. I was told it has something to do with your host though (not exactly sure how). I simply refresh the page to fix it.