Recently, I was trying to help somebody with deleting the uploaded files from their WP blog. After lurking through all the databases, unserializing strings, and checking some things. I found that the error was being caused by the path using backwards slashes in it. Quickly I set the path in the wp-config.php file, and it was ready to roll, and everything worked good again.
Because I could not find any information about this on the internet, and I did not see anything about this on the forum, I am posting it here. I could not find a bug tracker or anywhere else for it.
Here is the line I changed in wp-config.php:
and in my case, I had to change it to: