A PM thread was closed by @System after reaching 500 replies.
That would normally be fine, receipt there’s an issue with the math invoiced.
According to the automatic lock:
This message was automatically closed after reaching the maximum limit of 500 replies.
There are currently 501 posts in that thread.
Post 501 is the system message, while post 1 is the original post.
The first post is not a reply, so move all the numbers back.
So then the thread was closed after 499 replies.