If you have recently posted a message (or see a link to recently posted message) and get the message "an invalid post id was requested" - It is jost likely that the message has been posted recently but is still waiting for approval.

All messages that are posted are subject to moderation (as unfortunately people do regularly attempt to post inappropriate material)

This means that you may have to wait a while until your message has been read and approved by a moderator. This could mean that your message will not be public for a day.