Error on Multi-Language Shop when Plug-In 'String-Tanslation' is active

My shop is runnig in German and Italian, the normal translation i made via Poedit, but i'm not able to translate the Strings of the Marketpress-Plug-In like Messages. The problem is, that when i activate WPMLs 'String Translation' i am no longer able to access a single product page. I get a 404 message 'product not found'.

The Shop (wip) is online here:

At the moment the Plug-In 'String Translation', causing the error is deactivated...

Is this a permalink issue? How can this be resolved?

Thank you