I suspect that the variable ${file.name} is not substituted correctly. As a result, the value of log4j.appender.FILE.File becomes logs/. As such, Java tries to create a log file named logs/, but probably it is an existing directory, so you get the exception.
As a quick remedy, change the log4j.appender.FILE.File setting to point to file by absolute path, for example /tmp/mytest.log. You should not get an exception.
After that you can proceed to debugging why ${file.name} is not replaced correctly in your runtime environment.