It appears that at some point recently, Avast decided that the adware built into BitTorrent is a threat, and started completely blocking the app. I went into the settings, found the history of what was being blocked, checked with virustotal.com to see if other scans thought anything was dangerous, and then added exceptions to the Web and File System shields that made it work (basically putting in the server hosting BitTorrent’s updates, and the BitTorrent.app directory). But this is kind of annoying. Yes, BitTorrent includes some annoying adware, but they need to make money somehow. I registered for their ad-free usage already anyways. 
No different to Malwarebytes or Windows Defender and several other Anti-Malware programs, they all block the adware in Utorrent/BitTorrent.
Use a torrent client that doesn’t have adware such as Tixati.