Question : Copying file fails in one direction

I have 3 sites (let's call them Sites 1a, 2a, 2b).  They were connected by T1 circuits, but were recently converted to Metro-E.  Everything works great, except for when I copy files.  Normal traffic flows normally.  Here is the problem.  If I copy a file from 1a to 2a or 2b using just unc paths that were typed in windows explorer, it will start to copy and then at around 10 seconds left it will just freeze and finally get one of 2 errors.....1) Cannot copy XXXXX: The specified network name is no longer available or 2) the path is too deep .  The path is 21 characters, so I know it is not too deep.  If I remote in to the server that I'm trying to copy the file to at 2a or 2b using pc/anywhere and then copy the file across from that direction it works perfectly.  In both cases I can be on the machine at 2a and copy from 1a or be on the machine at 2b and copy from 1a.  But if I am on 1a, I can not copy to 2a or 2b.  The file I'm trying to copy is 4.8 MB.  Smaller files seem to work.  The 2a is running Windows Server 2003 and the 2b is running Windows Server 2000.  On the 1a site, I have tried a windows xp pro machine and a Windows Server 2003 server.  I have worked with the phone provider that installed the Metro-E and they finally brought in the hardware vendor for their equipment.  He is saying it is a windows problem and not theirs.  My only problem with that is it worked perfect on the T1 circuit, so even if it is a Windows problem I need some direction on what to be looking for.  I don't feel anything should need to be changed since the Metro-E is basically just a wired connection to another site.  Any one that could point me into the right direction or more things to try to narrow down the issue would be appreciated.

Answer : Copying file fails in one direction

What kind of hardware are you using?

Check your MTU's on all routers/firewalls to make sure they match.