The BES Client runs as the system account on computers and therefore does not have an associated user when it attempts to connect to a network share location. I suspect that you are having permission problems because when you run your script locally it uses your local user credentials to access the share point.
To get around this, you may need to use a Null Session Share which will allow anyone access to the files.
This article seems to describe how to set up a null session share for office patches but it should be relevant for you as well.
In order to bypass using it, I copied the file from the share locally and ran it locally without RunAsCurrentUser (same syntax as above) and it worked.
It is a little problematic when you need to run installation with relative addressing in that format from a share,
because you cannot download/copy the files to your local HD and run it from there (unless you preserve the tree)That was my original problem, and I did not find a solution for that yet…
combining RunAsCurrentUser with running a file from a share just does not work properly…