Subsites only give out 404

Hey dear WPMU Dev Support,
strangely all my subsites evolve in a 404 error. Permalinks have been resetet, but that didn’t help. Especially green-wood-international.com/impressum is bothering me right now, but of course others should work too. The .htaccess only consists of the basic WordPress entries, what else could it be?
Thanks for your help!
Tim