Weird error that goes away on machine reboot

I'm getting a weird error:

Warning: strpos() expects parameter 1 to be string, array given in C:\xampp\htdocs\wpmu-usspi\wp-includes\rss.php on line 577

There's no plug-in code running here but what's weird is that if I restart the machine, the error goes away -- but if I just restart Apache, it doesn't.

I have no idea how to deal with this one. It seems like WP just gets sick over time.