I managed to reproduce the bug Mikael Kleinwort reported above on the website of demo page of Telerik University (at your website).
1. Go to the Modules - > Generic Content management and create a new generic content.
2. Create a hyperlink and set the URL Address textbox to a page within the website. If you select an outside page (ie http://www.google.gr the bug does not appear). The hyperlink must be in the form: [Sitefinity]<GUID> (for example: [Sitefinity]8fde5e65-e549-4399-a8d3-c58e9b8acd37)
3.On the tooltip textbox write something in a non-english language. For example I wrote the greek word: Δοκιμή
4. Save and view the generic content item. The tooltip will become curropted.
This does not happen if you type an external site insteadin the Url Address Textbox.