Some days ago I had a problem with my ubuntu update manager, it was displaying a error message and the update manager was not working.
So I googled it and find the solution here.
Run in the terminal to search the broken repository
The broken repository should appear like this
Now just go to System > Administrator > Update Manager, click on button on the windows bottom Configuration, Click the Other Software tab.
Find the repositories which caused the failure on the update and uncheck them and then close the Software Centre.
Run, once again and should work:
Thats it folks