I know, plenty of threads about this, but not one with a sollution for me.
When using 3.0(.1) (network enabled) I had no problems upgrading plugins. When I wanted to upgrade to 3.0.2 I got the error from the topic title. I upgraded manually and didn't give it a thought. Now I get the same error when I want to upgrade a plugin. No, I didn't change anything but the few WP files that were changed.
So, the upgrade folder to 777, no success. wp-content to 777, same. wp-content and /plugins and /upgrade to 777, no succes. Deleted the upgrade folder, it gets recreated (740!), but the same error and in the end, deleted the plugin, tried to add a new one, same error.
I'm no fond of 777 permissions, I've always used 747 without problems.
Any suggestions I didn't think of?