Java serverapplication writing into Err stream instead of logfile

Question

I have a Weblogic server with Java application that is writing the log messages to a wrong file. Actually the logs (using Log4J) can be found in the error stream (the file /opt/APP/logs/app.err).

I've seen the issue on a weblogic server version 12.2.1.2.0 (but also someolder versions).

In the Server Startup parameters following is provided:

-Dlog4j.configuration=file:/opt/APP/config/app_log4j.properties 
-Dweblogic.Stderr=/opt/APP/logs/app.err 
-Dweblogic.Stdout=/opt/APP/logs/app.out 

In the code a sample line is:

private static final Logger LOG = LoggerFactory.getLogger(AppClassRunning.class);
...
LOG.info("Starting run");

I haven't found any manual configuration in the code that would change the redirecting of the log to the error stream.

In the file '/opt/APP/config/app_log4j.properties':

log4j.rootCategory                                     = INFO,fileA
log4j.appender.fileA                                   = org.apache.log4j.RollingFileAppender
log4j.appender.fileA.file                              = /opt/APP/logs/app_application_log4j.log
log4j.appender.fileA.layout                            = org.apache.log4j.PatternLayout
log4j.appender.fileA.Threshold                         = INFO 
log4j.appender.fileA.layout.ConversionPattern          = %t %d %-5p | (%c:%L) - %m%n
log4j.appender.fileA.MaxFileSize                       = 10MB
log4j.appender.fileA.MaxBackupIndex                    = 5
log4j.category.com.APP = INFO

I also tried to look for other statements in the code that directly write to the error stream - nothing found.
I tried to configure with the properties file to log somewhere else but without effect.

How can I find out (debugging seems not to work) what is the cause for writing to the error stream.