The dagger worked again for me after Weisshaupt, but only for a while. I went back and interacted with statues and opened the chests I've left for later due to the bug. But when doing Davrin's The Cauldron quest, the Evanuris altar at that area wasn't glowing and I can't interact with it. My playthrough is bugged again. So doing Weisshaupt is not a definitive fix, just like the people on EA forums found out. I assume you're already following the discussion, but here's the thread anyway.
I hope the next patch will be released soon, and if it actually addresses this issue, it will actually fix our saves and not have us require to start a new playthrough.