Seems like uneven sensitizing to me. How do you apply the second coat? (Hint: Are the marks show up exactly where you pour the second coating mix?)
a. Try to include a non-ionic surfactant in the coating mix (tween or... ilfosol maybe) for better absorption.
BTW, 3h between coatings seems excessively long:
b. (And/or) Try to apply the second coating right after the paper surface looses shine and become matte. (The paper should be wet but with no sensitizer pooling.)
c. Change paper.
Try a and/or b and/or c and see what happens.
Hope this helps,
P.S.1. 50g in 750ml fixer is a little bit on the strong side: 2 minutes in 2% is enough.
P.S.2. Selenium is not the best toner for Vandyke -> try gold or platinum or palladium before fixing; those are much much better...