Running a perennial trade surplus is unsustainable if the country belongs to an area with fixed exchange rates, unless all the other countries also run a big trade surplus.
German exports are not contained to EU, it's a major exporter worldwide.
That doesn't matter. If the rest of EU has a trade deficit the imbalances will still accumulate. The result of that is what we are currently seeing.
There is a case to be made that the trade deficits that the world runs against Germany is unsustainable.
This is more a problem of how rest of the world is governed than how Germany is.