On one of my Macs Spam Sieve was crashing when I trained an email message. I looked at the crash report and it mentioned that Mail Act-On had crashed. I looked in my bundles folder and I noticed 2 copies of Mail Act-On. I manually deleted the oldest Mail Act-On bundle and Spam Sieve starting working again.
I am not sure how two versions of Mail Act-On got in there but I thought I share this with the group in case it might help any one else.