It seems there is a bug with the wikis permalinks if a custrom structure is used.
1. Clean wordpress installation
2. Set permalink structure to Custom Structure /archives/%postname%/
3. Activate wiki plugin
4. Create wiki page and try to view
The wiki is not found.
If I skip step 2 (doesn't change) permalink structure I don't get any problems. A workaround seems to be to change permalinks to default and then back to Custom Structure. Then it works.