Hi Diana
That shows the OData Service URL you can test in transaction /IWFND/GW_CLIENT to see what is being returned. If you remove the /$count you should see the tasks being returned themselves or at least some reason why.
Frankly at this point it's clear something pretty basic is wrong. Could be missing authorizations, could be backend system connection, could be something else going on.
I'd suggest you raise a SAP Message to support and get some help in looking at it.
Don't forget to tell them what notes you've already looked at .
Rgds,
Jocelyn