Ockham (cf. this article on him by the Catholic logician Paul V. Spade) invented his dead-end nominalist philosophy in order to justify his being against the papacy (cf. Thomist John Deely's Four Ages of Understanding p. 394 ff., which shows how the Great Western "Schism" lead to the adoption of Ockham's nominalism, despite its weakness).
But was Ockham really a sedevacantist (i.e., one not explicitly against the papacy per se but against a particular pope claimant)?
The following quote from Salza & Siscoe's True or False Pope?: Refuting Sedevacantism and Other Modern Errors p. 210 is quite convincing.
Ockham wrote—"at the end of his letter to the General Chapter in Assisi in the spring of 1334" (cf. Tractatus de Successivis translation p. 12), defending his opposition to Pope John XXII, who opposed the (then-material) dogma that the souls of the deceased destined to heaven behold the Beatific Vision immediately after death, defined by John XXII's successor Benedict XII in Benedictus Deus—that:
Because of the errors and the heresies mentioned above and countless others, I turned away from the obedience of the false Pope and all who were his friends to the prejudice of the orthodox faith. For men of great learning showed me that because of his errors and heresies the same pseudo-Pope is heretical, deprived of his papacy, and excommunicated by Canon Law itself, without need of further sentence. … In proof thereof several volumes have been published. … For against the errors of this pseudo-Pope I have turned my face like the hardest rock, so that neither lies nor calumnies nor any persecution (which cannot touch my innermost self in any bodily fashion), nor great numbers of men who believe in him or favor him or even defend him, shall be able to prevent me from attacking or reproving his errors, as long as I shall have hand, paper, pen, and ink. …
If anyone should like to recall me or anyone else who has turned away from the obedience of the false Pope and his friends, let him try to defend his Constitutions and sermons, and show that they agree with Holy Scripture, or that a Pope cannot fall into the wickedness of heresy, or let him show by holy authorities or manifest reasons that one who knows the Pope to be a notorious heretic is obliged to obey him. Let him not, however, adduce the great number of his adherents, nor base his arguments on reproaches, because those who try to arm themselves with great numbers of lies, reproaches, threats, and false calumnies, show that they are void of truth and reason. Therefore let none believe that I mean to turn away from the recognized truth because of the great number of those in favor of the pseudo-Pope, or because of proofs that are common to heretics and to orthodox men, because I prefer Holy Scripture to a man unlearned in holy science, and I have a higher esteem for the doctrine of the Fathers who reign with Christ than for the tradition of men dwelling in this mortal life.
The Church never condemned Ockham's theories, although Ockham was excommunicated for leaving Avignon without permission.
W. Turner writes in the old Catholic Encyclopedia "Ockham" entry:
Ockham's attitude towards the established order in the Church and towards the recognized system of philosophy in the academic world of his day was one of protest. He has, indeed, been called "the first Protestant". Nevertheless, he recognized in his polemical writings the authority of the Church in spiritual matters, and did not diminish that authority in any respect.
This is sedevacantism because sedevacantism doesn't deny the papacy (as Protestants do).