Error updating primary drop. Mail left in temporary maildrop (28)

Hi All,

I'm new to this list, but I have quite an urgent matter which I need to
solve. 

If one of the users on my mail server tries to fetch its mail, I see the
follow error show up in the maillog:

Dec 14 13:08:34 cobalt4i in.qpopper[7293]: rzoeteman at xxx.xxx.xxx.xxx
(xxx.xxx.xxx.xxx): -ERR Error updating primary drop. Mail left in
temporary maildrop (28)

The server is an old cobalt RAQ server (4i) running qpopper 3.0. 
This user is unable to fetch all of the emails that are waiting for him.

Can anyone tell what this error means, since even Google hasn't got any
useful results.

Kind regards,

Zazell