Sometimes ... it's time to slap the head against the wall. :-)
Just a config thing:
My Plugin TraceSource was not writing to the correct listener...
The following config works perfectly, writing to both, file and console listener:
<system.diagnostics>
<trace autoflush="true" indentsize="4">
<listeners>
<add name="defaultLogListener" />
<add name="defaultFileListener" />
</listeners>
</trace>
<sources>
<source name="ImporterPlugin" switchvalue="All">
<listeners>
<add name="defaultLogListener" />
<add name="defaultFileListener" />
</listeners>
</source>
</sources>
<sharedlisteners>
<add name="defaultLogListener">
type="System.Diagnostics.ConsoleTraceListener"
traceOutputOptions="Timestamp"/>
<add name="defaultFileListener">
type="System.Diagnostics.TextWriterTraceListener"
initializeData="application.log" />
</add></add></sharedlisteners>
</system.diagnostics>