The main line of code that was changed was line 22 : ' $plugin = & JPluginHelper::getPlugin('system', 'plg_core_login_redirect');'
Which was referencing the wrong plugin completely. This incorrect piece of code would indeed report an error on the next line because it wouldn't have returned any results making line 23 a non-object.
Please double check to make sure that the fix I supplied included that code change at line 22.
Really, that is the only thing I can see that could generate the issue reported.
Unfortunately, as I said, this version of the plugin isn't really supported any longer. Furthermore, I don't even have a dev version of Joomla 1.5.x to further test.
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Update: I realized just before posting this that again, I had included an incorrect line of code. Line 22 should actually read:
$plugin = & JPluginHelper::getPlugin('system', 'core_login_redirect');
This change has been made and posted to the downloads on our website, you may try it again. Hopefully this will finally resolve your issue.
I'd love to change the world... but they won't give me the source code!