Same problem here. Trying to assassinate Primate Artorius does not work and so I can't finish the quest line. I tried it with all addons on then all addons off but it didn't make a difference. There does seem to be a pixel where the synergy/blade of woe sign quickly shows up then disappears again.

I noticed after the big patch that it had become extremely finicky to use the blade of woe but now, after the "hotfix", it's gone completely. I tried the repeatables for the dark brotherhood and the synergy wouldn't show up for any of them. That was not all that good but could be accepted. Not being able to assassinate the primate artorius and so not being able to finish the dark brotherhood main quest line is not really acceptable...it is, after all, the main questline for that guild.