Opened office 2003 files can not be saved on windows 2000 server

[sufkont]

Version: 2008 Operating System: Mac OS X 10.6 (Snow Leopard) Processor: Intel Hello all

I have office on my mac in a windows network and save al my files on a windows 2000 server.
When I open a 2003 file from the server, I can not save it back to this server.
All other servers and iDisk and Dropbox works fine
When I change the file with "save as" to a 2007/2008 format the file can be saved on the windows 2000 server without any problem.

What can be the problem?

thanks a lot for your help!

Marco