Server: 14.2.1 Client: 14.0.2 and 14.2.1

I haven't been able to find anything on this variation of the D109 error.

Here's the body of the alert:
Error that occurred: D109-Bad parameter
Type of message: Flush personal address book
Message passed to server by: fred
To requeue, save the last attachment to <post office path>\WPCSOUT\OFS\0

fred is in PO1 and the alert is coming from PO2's POA. I don't know which client fred uses (probably 14.0.2 as we've only just begun to roll out 14.2.1).

There are about 20 user accounts (out of 5000+) that account for all D109 alerts.

Any suggestions as to what is causing this?