HTMLLayout and LayoutWrappingEncoder are producing unusuable HTML output, the HTML file isn't closed by </table></body></html>. When you are using the append element, a new logging event produces the complete html header, headlines and table header.

Configuration:
<appender name="FILE" class="ch.qos.logback.core.FileAppender">
<file>../../logs/server_nodes.html</file>
<encoder class="ch.qos.logback.core.encoder.LayoutWrappingEncoder">
<layout class="ch.qos.logback.classic.html.HTMLLayout">
<pattern>%d

%msg</pattern>
</layout>
</encoder>
<append>true</append>
</appender>

<logger name="file.logger" additivity="false">
<level value="info" />
<appender-ref ref="FILE" />
</logger>