Moinmoin,
in order to avoid MIDI loop, You must break the connection:
... P'08 MIDO out -> Cubase MIDI in -> Cubase MIDI out -> P'08 MIDI in -> P'08 MIDI out -> Cubase MIDI in...
either at the Cubase or at the P'08 side.
Everything seems to be OK if You break the loop at the Cubase-side, as You wrote:
Everything works OK when turning MIDI Thru off and P08 in Local On.
Nothing coming in from P'08 to Cubase will be sent back to P'08.
To break the loop at P'08, set local control (in P'08 global settings) to Off. Nothing coming in from Cubase to P'08 will be sent back to Cubase. Don't forget to set P'08 global settings regarding MIDI parameter send / receive and MIDI program to Your purpose.
Every keypress, knob tweak, etc. will be sent from P'08 to Cubase and echoed back to P'08 sound modules, which will behave like they were internally connected as long as Cubase is present. If this is Your standard MIDI setup, everything is fine.
Personally however I prefer breaking the loop at the sequencer's side because I use my P'08 sometimes with, sometimes without sequencer. If I would break the loop at P'08 I would have to change its global settings every time I switch from or to sequencer use (which I might forget...).
There may be a 3rd method to break the loop within the USB to classical MIDI converter, breaking the loop here virtually pulls one MIDI cable at the P'08 (which obviously is the 4th method), so I would leave that untouched.
HTH
Martin