We need more information, including log files.
Sonarr version (exact version):
Mono version (if Sonarr is not running on Windows):
(Make sure debug logging is enabled in settings and post the full log to hastebin/pastebin/dropbox/google drive or something similar, do not post them directly here)
Party of 1-2-3 and
tvdb:342025 find a result for me.