One of my user is also getting the same error. I have installed document
library from SPS V2 Beta,but no use.

Backward compatible library and client components are a part of SPS
installation kit.

Has anyone tried the suggested scenario: using the SPS2003 *client
components* for the backward compatible document libraries against an
SPS2001 server?

I suspect it should work but would like to hear from someone how has
actually tried it.

I haven't had a lot of success with it. I'm currently managing a 2001
production environment and building my 2003 one in development.

The 2003 one works fine and I can check in/out etc. from Word and work
with the web folders without any issues. Development of web parts in
2003 isn't working 100% yet (problems creating new Web Library projects)
but this may be caused by the fact my development environment is in a
different domain than my desktop (which will change in the next couple
of weeks).

Managing the 2001 workspace doesn't work very well. While I can open up
the web folders and move files around, the properties menu doesn't come
up now so I can't use it to apply security or check documents in/out
from the web folders (it produces an error but doesn't say what it is).
My workaround is that I connect via terminal services to a dev server
running my 2001 development environment and use the 2001 client tools to
manage the 2001 production server. Not the best setup but it works.

Here's one data point from an SPS colleague:

"...when Office 2003 is installed the client components that are needed to
access the SPS 2001 server are removed. My response at this point would be
that it is best to use the client components from SPS 2003 - as these will
be the bits that will be updated (Hotfixes, SP's, etc.)."

Another authorative response from another SPS collegue:

"The SPS 2003 client components were tested against SPS 2001. I would
definitely recommend installing the SPS 2003 client, it contains all of the
security fixes and service pack fixes over the last two years. The SPS 2003
client will be available for download from the web."

Has anyone found the download url for the 2003 client??

For backward compatibility??????????

It is on the Office 2003 cd.

see this KB article

support.microsoft.com/default.aspx
&Product=off2003