qBittorrent v4.2.1 was released.
Due to libtorrent fixes, UDP through proxies should work again. In some cases it also caused crashes.
ATTENTION: There's a slight change in the way network interfaces are configured. If you have a specific network interface/local address set in the Advanced Settings, go and make sure that your settings have stayed the same. The extra setting for Listen on IPv6 address has been removed.
v4.2.1 changelog:

• FEATURE: Enable portable mode if "profile" directory exists (Tester798)
• FEATURE: Enable "Apply rate limit to peers on LAN" option by default (Chocobo1)
• BUGFIX: Sync translations from Transifex and run lupdate (sledgehammer999)
• BUGFIX: Don't unnecessarily delete OS files in folders (sledgehammer999)
• BUGFIX: Use the incomplete folder where appropriate (sledgehammer999)
• BUGFIX: Align Properties tab bar correctly on window resize (Prince Gupta)
• BUGFIX: Rework the listening IP/interface selection code (sledgehammer999)
• BUGFIX: Fix inconsistent icon for deleting torrent (Chocobo1)
• BUGFIX: Show torrent error message in transfer list (Chocobo1)
• BUGFIX: Fix stuck in wrong torrent state (Chocobo1)
• BUGFIX: Expand single-item folders in torrent content (warren)
• WEBUI: Bump Web API version (sledgehammer999)
• WEBUI: Add ability to rename torrent files from the WebUI (Thomas Piccirello)
• WEBUI: Mention lack of HTTPS in WebUI magnet link warning (nl6720)
• WEBUI: Fix HTML elements size in search tab (Chocobo1)
• SEARCH: Fix incorrect translation displayed after language change (Chocobo1)
• SEARCH: Fix missing translations in search plugins dialog (Chocobo1)
• WINDOWS: Update russian translation of the installer (Andrei Stepanov)