Let the dough sheets rest at room temperature for about 15 minutes. For the filling, mix quark, semolina, eggs, sugar, vanilla sugar and cinnamon. Peel, quarter, core and dice apples. Rinse raisins hot, dab dry. Stir apples, raisins and pumpkin seeds into the quark mixture.
Heat butter. Roll out the dough sheets on a damp tea towel. Brush 1 pastry sheet with butter and place it in a greased springform pan (26 cm Ø), dust with flour. Press down slightly at the edge, let it hang over the edge of the tin. Spread another 5 pastry sheets with butter and place them slightly offset on top.
Sprinkle the base with breadcrumbs and spread the apple mixture on top. Fold over the overhanging dough. Spread the remaining 4 pastry sheets thinly with butter and lay them loosely on the filling. Bake in the preheated oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer) for about 1 hour.
Place the strudel cake on a cake rack and immediately remove the edge of the tin. Let the cake cool down and dust with icing sugar. Serve with vanilla ice cream.