How to configure logging for server and client side?

Hello,

in connection with [Thread about mysterious behavior]
(https://vaadin.com/forum/thread/17758856/errors-while-using-vaadin-application-how-to-find-them) in my Vaadin application I want to activate a Vaadin wide logging (that means server and client side) using Log4J. But unfortunately I had no success.
To activate logging for GWT/client side I found something like this to be configured in widgetset.gwt.xml:

<inherits name="com.google.gwt.logging.Logging"/>
<set-property name="gwt.logging.simpleRemoteHandler" value="ENABLED" />
<set-property name="gwt.logging.logLevel" value="INFO"/> <!-- SEVERE, WARNING, INFO, CONFIG, FINE, FINER, FINEST -->
<set-property name="gwt.logging.enabled" value="TRUE"/>

But something is missing or wrong. During runtime I get lot of error messages like:

com.vaadin.server.VaadinServlet          : Requested resource 
[/VAADIN/widgetsets/myApplication.vaadin.widgetset.MyApplicationWidgetset/remote_logging] not found from filesystem or through class loader. Add widgetset and/or theme JAR to your classpath or add files to WebContent/VAADIN folder.

What is wrong? Do I have to configure something else?
For server side I found something like this:

com.Vaadin=FINER

But where I have to place it? I think, it is a Java logging stuff, or?
Or is there something else? I am using Vaadin 8.

Thanks in advance