It looks like your old queued posts had a string for a category (name perhaps?) instead of an id. This patch handles that case. I’ve backported it to beta, so updating should fix your instance.
I’m not sure why I didn’t see this is any of the test databases I ran it on.
https://github.com/discourse/discourse/commit/13c6bf54d0d08363ac36b0755a7a26386b6768e6