I've seen a similar thread come up in the GW2012 forums, but it doesn't appear to apply to my situation.

I have a single user who has been generating D109 errors from her caching mailbox. This is what I get in my admin mailbox several times per day:

Error that occurred: D109-Bad parameter
Type of message: DOWNLOADED status
Date/Time: Thursday, 11 February 2016 8:58am
Type of message: Messaging update
Message passed to server by: <redacted>
To requeue, save the last attachment to <post office path>\WPCSOUT\OFS\0

Here's the config:

GroupWise Server: GW2014.2 on SLES11. MTA and POA on the box. No antivirus/antispam, that's all handled at the client or pre-gateway level.
Client: GroupWise 2014.2 on Windows 8.1 (updated).

This only occurs when she is in caching mode. If we switch her to online mode the errors go away. We've rebuilt the online mailbox, in addition to daily structure and weekly contents checks we've also done standalone GWCheck checks on this individual mailbox, so far as to run a structural rebuild. The caching mailbox has been deleted and recreated (post GWCHECK) but we're still getting the same result.

This is only affecting one of about 250 users with the full client (I also have 900+ webaccess only clients).

Any thoughts?