Thanks for the suggestion 
Looked in to this now, it seems probably not related as a couple of users that didn’t have any messages listed under messages → Inbox/Send/Archive still got errors, as well as for a user that had a fair few messages.
Weird is that now it’s working sporadically some of the time, on some users. Prior to the update, it worked every time without issue.
Note: From the review queue, it seems that agree → delete spammer is working fine currently (deletes all posts and the user).