I have an F8000. After signing up for the first time, iPlayer just showed me a black screen. This worked for me:
Press Menu, Select 'Smart Features', find and select 'Reset Smart Hub' - enter your pin (see other threads on what this might be - I knew mine :o) ). Wait.
It will remove installed apps and accounts... and re-add its 'required' ones - I ended up with some new ones, as well as 'changed' ones. iPlayer has since worked a number of times.