I have a similar issue, Postbox hangs when I select a message and select “Junk.” Postbox 5.0.14, SpamSieve 2.9.28 on MacOS 10.11.6
I can view or delete message, but not flag as “Junk.” Postbox freezes with spinning beachball. This behavior happens only with spam in my Inbox. If I click on any other message, say from a regular correspondent, I can flag as “Junk” without issue.
Thanks for sending the sample logs. It looks like Postbox is hanging in its own internal code (not SpamSieve or the SpamSieve plug-in or the OS). So I recommend reporting this to the Postbox developers.
Yes, it looks like it hung before (or after) communicating with SpamSieve.
I would be interested to see those, too.
Also, you could see whether the problem occurs with a new profile. From the General preferences, you can set Postbox to show the Profiles window at launch.