login.php and includes/functions_login.php are replaced by that patch (but may not have actually changed if you already had PL3), so maybe make sure those are OK - but you said you already tried reuploading.
Maybe if you have SKIP_ALL_ERRORS defined in config.php try temporarily removing that to see if it displays anything? I'm just thinking "out loud", it sounds like you do this stuff a lot more than I do.
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