XCOPY - errorlevel code 4: Help please!

Hello, I'm new to this so would very much appreciate any help.

Background:

I'm running win vista and I think DOS V6.0.6000.

My problem:

I have a batch file and within it I am trying to backup my 'Documents' folder but I keep getting an errorlevel code 4 on the xcopy.

I would be grateful if anyone could tell me why this is and how can I get around it please?

Commands I'm using:

SET src_folder3=%homedrive%%homepath%\Documents
SET backup_folder=E:\Temp1\%backup_date%

xcopy %src_folder3% %backup_folder%\Documents /s /e /y /g /h /r /k >>%backup_log%

Additional Info:

When I replace 'Documents' on the xcopy line for another folder (e.g. Desktop, Favorites, etc...) no problem occurs.

I don't think that the command line itself is the problem but more likely something to do with the 'Documents' folder or it's contents/sub-contents.

Help please!