Yes, if the recipient field is 0 then there's nothing you can do.
That "1" trick will only work with the Sender field.
You can try restoring from a backup and try again.
If you do leave it, then you can remove any of the "0" PMs and Address Book entries as they are unneeded.
If you restore and try again, try setting the maximum execution time higher in the php.ini file on your server. What happens, is if there are too many PMs to process, the server seems to time out and/or start over, which causes the 0 effect. If you are on a shared server, you probably won't have access to this.
Infopop's latest official advice is to remove as many old PMs as you can. Perhaps install the hack from here that allows users to transfer their PMs to them via email... and ask them to do some housecleaning before you blow out any messages over x days old.
Hope that helps.
