If you receive an error that reads "The entry has not been posted due to this protocol error: Client error: Don't have access to requested journal" when approving an entry, it indicates that the author of the entry no longer has posting access to the community you are moderating. This usually occurs because they leave the community after posting the entry but before it is approved. This will also occur if a community maintainer removes the user's posting access through the Community Members page. You may either contact the user and ask them to rejoin the community, or grant them posting access if they are still a member. If you just want to remove the entry from the moderation queue without having the member rejoin or grant them posting access, you will need to reject the entry.
I'm thinking this would work as the last paragraph in the FAQ (especially since I can't think of anywhere better to put it).