There shouldn't be a problem with your sitename. I've tested it on my test server and it works correctly.
Make sure that your System - ARTIO JoomSEF plugin in Joomla is ordered as the first of system plugins. If that doesn't help, try temporarily changing the "ü" in your sitename to "u" just to check if it causes the problem. If that doesn't help, there's probably some interference with another plugin setting the page title.
Also make sure that your JoomSEF is set up correctly - check that everything is enabled in JoomSEF's Control Panel in the SEF Status box on the right side.