well, I have no answer ...
here is the complete article I found on Metalink, which is the only one referencing your errors :


Problem Description
-------------------

Trying to use the UNC (Universal Naming Convention) for a path and
file name in SQL*Loader produces results similar the following:

sqlldr scott/tiger@local data=\\somehost.abc.com\x\test.dat
bad=test.bad log=test.log control=test.ctl discard=test.dsc

This fails with the following errors:

SQL*Loader-500: Unable to open file (\somehost.abc.com\x\test.dat)
SQL*Loader-553: file not found
SQL*Loader-509: System error: The system cannot find the file specified.


Solution Description
--------------------

There are problems with how SQL*Loader parses path names trying to use
UNC. The work around for this is to invert the UNX path separators.
To get the desired effect, you will have to use the following:

sqlldr scott/tiger@local data=//somehost.abc.com/x\test.dat
bad=test.bad log=test.log control=test.ctl discard=test.dsc

You can use this for any of the parameters requiring a path and file
name as in the following parameters:

data=//somehost.abc.com/x\test.dat
bad=//somehost.abc.com/x\test.bad
log=//somehost.abc.com/x\test.log
control=//somehost.abc.com/x\test.ctl
discard=//somehost.abc.com/x\test.dsc

Note: "somehost.abc.com" represents the Windows networking computer name.


References
----------

[BUG:1346942] UNC PATH NAMES DO NOT WORK PROPERLY WITH SQL LOADER