Plugin update Issue: Could not find update in transient, or filesystem permissions

Previously I used to get a 'source not found message' when trying to update plugins. I was asked to manually update the WMUdev plugin. Which I did.

Now I keep getting the message: Could not find update in transient, or filesystem permissions

Permissions on the uploads and plugins folder are set to 0755

Please advise as I cannot keep deativating, deleting and uploading important plugins manually all of the time.

Note: There is no issue with installing new plugins via the dashboard, only in updating them.