No log output despite various debug prefs and app.update.log = true

I am trying to debug a failing update process on OSX but I don't get any decent log output indicating what's going wrong. 

Added this prefs:
/* debugging prefs */pref("browser.dom.window.dump.enabled", true);
pref("javascript.options.showInConsole", true);
pref("javascript.options.strict", true);
pref("nglayout.debug.disable_xul_cache", true);
pref("nglayout.debug.disable_xul_fastload", true);
pref("app.update.log", true);

And running the app with "-jsconsole".

But despite this, the app starts, the update gets installed, in a wrong way, leaving the app broken, and there is no output anywhere to understand why it's malfunctioning. 

The update.log is there, but I am interested in how updater is invoked in the first place, since I suspect that the second command line argument is wrong (the destination directory). See https://groups.google.com/forum/?fromgroups=#!topic/mozilla.dev.tech.xul/rdal5c2kzTU

Any tips appreciated! This is driving me nuts :P