we tested it, and it’s really very strange. Have you installed plugins? Or changed something in the php files?
You can try also this: go to WordPress admin -> Settings -> Permalinks and set it to default, save it and test it, normally now it have to work. Then set it again to the custom permalinks.
You can us send a login for the website ( info @ 7theme . net ), then we can have a look at the settings and help you to find the problem.