Access Denied error partly through copying a file

We have 3 offices which are linked by 3 permanent VPN tunnels over DSL
services, two of which are SDSL (Site A & B) and the other office is
ADSL (Site C).

The problem we are having is that when a Server (and some desktops) in
Site A or B try to copy a medium size file to Site C, around 100kB in
size it will get part of the way through then receive a "Cannot copy
file blah Access Denied", if we then try a smaller files say around
50kB we do not get this problem, it will copy the file and it is all
fine.

All 3 Sites have Windows 2003 SP1 with the latest updates except the
ones released on 14th as I have rolled them back as I thought they
might have been the cause of this issue. We also have the hot fix
http://support.microsoft.com/?kbid=898060 which caused us a lot of
grief in the past.

We have tried the same thing from two of our desktop PC's using are
Domain Admin account one of the machines work fine, we can send MB of
data without failure consistently the other desktop will fail the same
way as the Servers. This has taken the focus away from the router
causing problems because if it was the router then that desktop should
be having the same problem as everything else.

Copying between servers in Site C we have no problem we can send GB of
data with no failures at all.

Copying the same medium sized file from C to A or B works fine.

Copying between Site A & B servers and desktops we have no problems. It
is just when some machines try to copy from Site A or B to Site C.

This problem was first noticed when our DFS in Site C stopped
replicating data, about a week back. As far as I can tell this
replication problem started a week before that and as far as we know
the last things to change was the application of the Updates KB911564,
KB911927 & KB913446, before that our systems have been running
perfectly for about 3 months with no issues reported.

I have completely run out of ideas as to what is causing this problem
and any input to find a solution will be greatly appreciated.

We have resolved this issue, for the help of a future person with a
similar problem. The cause ended up being an MTU setting on Site C's
router. This router was configured to an MTU size of 1452 and our other
2 routers were configured to an MTU size of 1500.

This is a very odd problem as this router has been configured this way
since 2003! And secondly we had one XP machine that could copy files
without issue further blurring the problem device for us.

We suspect that Site C's ISP has changed something internally to loose
their previously required setting of 1452.

Anyway that is what it was for us.