Sweep return code 8, could not open file

Hi everyone,

I am currently in a pickle here, running out of ideas, and hopefully someone encountered a similar, or same problem as I had.

Environment:SUSE Linux Enterprise Server 11 (x86_64) 

I am running an Java application on Websphere Application Server 8.0.0.10, which uploads a file from the GUI of the Java application into a folder,
and then executes the system command  "/usr/bin/sweep <FILENAME>  -remove -nc -s".  
(sweep is a shortcut to /opt/sophos-av/bin/savscan, and both files have rwx permissions for user/group/other)
When the sweep ends, it always gives return code 2, or extended return code 8, which according to the user manual means "A survivable error occurs".

The output of sweep says, that it could not open the file, and it looks something like this:

...
Could not open <FILENAME>
0 files scanned in 5 seconds.
1 error was encountered.
No viruses were discovered.
...

This should not happen, because the file was just created, and the user executing sweep is the same user with which the file was saved on the hard drive.

Further information:

The Websphere Application Server is not run by root, but another user, who will be called wasuser here, who belongs to the wasusergroup.
The folder in which the file is uploaded belongs to this user, and it's group. Only the user and the group have rwx permission on the folder.
When I run the exact same command while logged in as the wasuser in a putty session, I get no errors, and the file is scanned without any problems.
I find this strange, because as far as I know, it should be indifferent whether I am logged in as wasuser in a putty session and execute sweep,
or the websphere application server instance, started by wasuser, is running sweep.

My questions:

Why is sweep not able to open the file, if it was executed from a Websphere environment?
Why does sweep have no problem scanning the file when it is run manually?

What I expect is, that sweep finds and scans the file, which currently is not happening...

Thank you in advance!