can you check via SWI5 in backend for the user where these items are lying?
So in SWIA, you can see the agent correctly assigned? ie approver?
are you using UWL or POWL? please check if the portal is connected to backend system correctly ie thru UWL? if you are using UWL etc and getting the workitems
does it fail for only this type of workitem or all types of workitems, then you need to check your connection to the backend