Vaadin 6 to 7 migration - UI provider error

Vaadin 6 / 7 / 8
1

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
20913.png

2

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
https://vaadin.com/book/-/page/application.environment.html#application.environment.web-xml
 for more details.