It's one particular feed that's doing this; its unread count increases over time, and some of its items show up if I hit "all sites," which lets me work the unread count down to 8. Those eight seem not to be visible by any means, though, if they exist. Refreshing has no effect, and it occurs on multiple machines, and in the Android app.
I just posted something about a similar experience, but the folder the feeds are in is saying "no stories to read" when there are 175 unread stories in the individual feeds. I can click on the individual feeds and read the stories there, but the folder they're all in just gives me the goose egg.
I posted this in another thread, but it was a bug I introduced 48 hours ago and fixed 24 hours ago. It will work it's way through the system, but there may be residual blank stories left. Get 12 of those in a row and it shows nothing (as is the case here). But it shouldn't occur on new stories, and if it happened to old stories then those are likely to be wiped away by newer stories. It's likely that this may be seen again, but it should resolve itself.