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The upgrade to 2.5 went smooth for my community, but I have weird issues with unread state:
The categories and all discussions pages never seem to match up; clicking "all as read" usually fixes it, but this is a bit confusing.
Posts that are somewhat old but not too old seem to have been marked as unread, can't be flipped to unread automatically with the mentioned feature, and don't show up on the categories page as making the categories have unread items. If I manually read them, they're marked as read.