Try a 10046 trace on an execution.
eg, I created a basic run_host_command java as per examples on AskTom, and then did:
SQL> alter session set events = '10046 trace name context forever, level 12';
SQL> exec rc('C:\bin\ffmpeg\bin\ffmpeg.exe')
PL/SQL procedure successfully completed.
But in the trace file, I see this:
java.security.AccessControlException: the Permission (java.io.FilePermission C:\bin\ffmpeg\bin\ffmpeg.exe execute) has not been granted to MCDONAC. The PL/SQL to grant this is dbms_java.grant_permission( 'MCDONAC', 'SYS:java.io.FilePermission', 'C:\bin\ffmpeg\bin\ffmpeg.exe', 'execute' )
at java.security.AccessControlContext.checkPermission(AccessControlContext.java)
at java.security.AccessController.checkPermission(AccessController.java)
at java.lang.SecurityManager.checkPermission(SecurityManager.java)
at oracle.aurora.rdbms.SecurityManagerImpl.checkPermission(SecurityManagerImpl.java)
So check the trace file - it might give you some leads.
If that doesn't help, then add as much debugging into the the PL/SQL *and* the Java (eg write to debug file) as possible, and see what you can track down.
Hope this helps.