Thanks for the report. It looks like SpamSieve is encountering errors reading the messages from Mail. Please try updating to the SpamSieve 3.0.1 beta, as I think this may fix the problem or at least report a more useful error message.
Also, if you have renamed your accounts in Mail, please go back to the Select Mailboxes to Filter… sheet in SpamSieve and re-select the mailboxes that you want to be filtered.