Catalyzes the hydroxylation of 2-octaprenyl-3-methyl-6-methoxy-1,4-benzoquinol during ubiquinone biosynthesis. a 6-methoxy-3-methyl-2-all-trans-polyprenyl-1,4-benzoquinol + AH2 + O2 = A + a 3-demethylubiquinol + H2O Cofactor biosynthesis; ubiquinone biosynthesis. Component of the Ubi complex metabolon, which regroups five ubiquinone biosynthesis proteins (UbiE, UbiF, UbiG, UbiH and UbiI) and two accessory factors (UbiK and the lipid-binding protein UbiJ). Belongs to the UbiH/COQ6 family.