There's another thread about this. One poster gave a tip for opening these by selecting the forward option for the message from the list of all PMs in your inbox. Here's what mine said:
WebmasterAlex wrote on 10-28-2004 06:32 PM:
In the next several weeks we will be upgrading the boards to a new version of V-Bulletin. As part of that process private message storage will be temporarily reduced, probably to one week. I am giving everyone advanced notice so they have time to copy and past PM's that they need to save.
Thanks
Alex