I don't know if the "?" is a problem, but I'm having what may be a similar problem.
I'm trying to redisplay a couple of feeds using WP 2.1 and the RSS widget. I've been using the feeds for months and haven't really noticed any problem in other tools I've used to display them.
What's happening is that more than half the time I seem to get an error ("An error has occured; the feed is probably down. Try again later."), but if I click the RSS icon used to subscribe to the feed, it comes up just fine.
[Actually, I'm trying to display 4 feeds. Feeds 3 and 4 seem to be rock solid -- just the first two seem to be having trouble.]
This WP 2.1 installation is less than 24 hours old, so I don't have a lot of history here. But if there's something that might make the feed connections unreliable, I'd be interested to hear about it.
In the meantime, maybe I'll look into learning more about the widget architecture so I can do something other than treat them as a black box.