In his essay Materials for Writing, and Forms of Books (available online in the 1978 History of Books and Printing edited by Paul A. Winckler), Falconer Madan reiterates the claim that a paper mill was established in Mainz, in 1320
. In Germany the first factories seem to have been established between Cologne and Mainz towards the end of the thirteenth century, and in Mainz itself about the year 1320.
In From Paper-Mill to Pressroom by William Bond Wheelwright and published in 1920:
The oldest-known document on cotton paper is a deed of King Roger of Sicily, dated 1102. It is probable that the famous mills of Fabriano sprang from Sicilian sources; their establishment was followed in 1360 by a mill in Padua, and later in Treviso, Bologna, Palma, Milan and Venice, while the first paper-mill of Germany was that of Ulman Stromer at Mainz in 1320.
At least one error in the preceding quote - Ulman Stromer was not the founder of an Mainz papermill in 1320, because: he was born in 1329, in Nuremberg, and kept a diary chronicling the founding of his Papermill in Nuremberg in 1390.
From Scribes, Script, and Books: The Book Arts from Antiquity to the Renaissance, by Leila Avrin :
In the fourteenth century, papermaking spread northward. By the 1320's there were mills in Cologne, Augsburg, and Mainz, and in 1390, Italian papermakers operated the Stromer Mill just outside Nuremberg.
The provenance of Ulman Stromer's paper mill in Nuremberg, founded 1390, is well established from his (extant) diary. There are conflicting claims of earlier mills, in the Rhineland, predating Stromer's by 70 years; however I can find no corroborating evidence of these claims beyond vague references in 20th century documents. I will give the last word to The Gutenberg Museum in Mainz, who one might expect to have a bias (if at all) favouring a Mainz claim:
Documents tell us that the first paper mill in Germany was Ulman Stromer’s Gleismühl in Nuremberg which started manufacture in 1390.