I have not received any other reports like this, so for now I’m assuming it’s a localized problem.
What do you mean by “SpamSieve won’t load”? Do you see it start to load and then stop? Or do you simply not see it in your Dock? Please note that SpamSieve is not supposed to launch when you launch Mail. It only auto-launches when Mail downloads new messages that need to be filtered or when you train messages as spam or good.
This message concerns SyncServer, a program that Mail and Safari use but SpamSieve does not.
I would not expect it to, since the crash message is for SyncServer. Perhaps once SyncServer is restarted, Mail can finish doing what it needs to do before it can check for new mail. In any case, I don’t see a direct relation to SpamSieve.