To reproduce, using release 1.35, create a form with an uploadable field, and upload a file with it, specifying a license.
Then an error Using $this in static context will appear.
It affects, at least, lines 483, 485, 491, 493 of PFUploadWindow::getInitialPageText(). Perhaps, this method should not be static.
UPD: also affected: PFUploadWindow::getExistsWarning(), lines 637, 640, 642, 646, 652, 668.