BOINC just uploaded to GPUGRID two WUs "GERARD_A2AR_OSTRUC_NUL2-0-2-...", and when I looked them up in my results, a "confirmation error" is shown for both of them.
Any idea how this could happen? In fact, I can't remember having had this before.

Your host has been duplicated somehow. I suspect the incident caused this duplication responsible for these errors.

The cause for the host "duplication" most probably was that I had to restore the system this morning after a crash last night. I realized only later that this PC got a new number (321839 vice 307671).

What also happened: when I started the BOING Manager, and under "Projects" - "GPUGRID" klicked on "Update", I receive a BOINC Manager notice saying:

Your app_config.xml file refers to an unknown application 'acemdshort'. Known applications: None.

This was happening only once though, no more later.

Trying to make sure that these "confirmation errors" do not happen again, I removed GPUGRID from the projects and added it new (the above cited notice this time did NOT show up).

2 new WUs were downloaded half an hour ago, so I'll see tomorrow morning whether now all works well.