I have the same issue on both Worpress 4.9.8 and 5.0.1 with Classic Editor 1.3 on Chrome. It works fine on Safari and Firefox but not in Chrome (71.0.3578.98) in normal or incognito mode.
ETA: Updating to 5.0.1 with Classic Editor 1.3 breaks it on Safari as well. Will not try on Firefox or I’ll run out of browsers to use.
