I have connection tracking enabled and use mangle to mark connections. In firewall → connections I see many connections with connection mark shown as (unknown 13). I definitely have no such mark. What is this unknown connection?
That usually indicates that the connection was marked by some mangle but then all rules naming or using that mark were deleted and so the textual label for mark #13 was removed, thus the connection list has no way to translate the value 13 to text.