While approving the requests in UWL(universal worklist) under Tasks tab frequently ( every 5 or 10 minutes),uwl throughs
1)Unable to perform the request.Try again and problem persists,inform your system administrator.
2)The item is no longer valid.Select another item or refresh the item list view.
NW-04s,EP 7.0 with SP15
We had a similar problem. In our case the problem was a corrupt XML file. I'd try making a backup of your main UWL config XML file (forget the name now), and then letting the system re-create it. Do the same for the XML files of the backend systems that display the item you are having a problem with... Maybe that will help.
The only other reason (that I can think of) that an item would "no longer be valid" is that someone already did something with it on the backend.