Guillaume,

  I had similar problem where by backups were aborting with the error
filename too long.  NT has 256 characters limitation with filenames.
  If you are do not have macintosh support on your NT server, make sure you
have you have USEUNICODEFILES NO in your dsm.opt.  This should solve
 the long filename error.
    I opened a call with Tivoli and they were not able to provide me with
any concrete answer, their suggestion was to go to 4.21 client.

Hope this helps,

Kelil