Vaadin 6 to 7 migration - UI provider error

I am migrating from Vaadin 6 to Vaadin 7. After deploying to jBoss 7.1.1, I can open the page, but there comes an error
(see attachment)

Does anyone know what that means in general and how I could possibly solve it?
Thanks in advance

When the Vaadin servlet gets a new request, it tries to find a UI to render. The UI can be defined by the UI parameter for the servlet, or by the UIProvider parameter. Check
for more details.