I finally got out my Mimi Sheraton book called just German Cooking and her potato dumpling recipe calls for "1 to 1 1/2" cups flour for two pounds of potato and two eggs when using already-cooked potato.
Another recipe for potato dumplings combines 1 lb cooked potato and 2 lbs raw grated potatoes and no eggs at all. It includes instructions to cube a large day old roll and saute the pieces until golden in hot fat, then encase each in the potato dough. And a third combines 3 lbs potatoes, peeled and grated, with a cereal made from 1 cup milk and 5 tblsp semolina (farina or cream of wheat cereal). Again no egg. In this one, the cooked cereal, cooked almost like a choux pastry (until it forms a ball and leaves the sides of the pan) is mixed with the grated potatoes from which the dumplings "the size of medium potaotes" are then made, and each gets a bread crouton stuffed in the center. Interesting technique which I am guessing reduces the density of the center and insures that the dumplings cook all the way through.
Would be interesting to make and compare them all!
Anyway, somewhere in the various discussions she suggests that if the dough is too wet/loose, adding bread crumbs until a workable consistency is achieved. That would have solved your problem, I think.