were
When two subjects are joined in a sentence by the correlative conjunctions "neither... nor" (or "either... or"), the verb must always agree with the closer subject (the one nearest to the blank).
Let's break down your specific sentence:
"Neither the editor (Subject 1) nor the contributors (Subject 2) ______ willing to accept the changes unconditionally."
Because "contributors" is plural and sits right next to the blank space, the verb must also be plural. Therefore: