Semi-good news !
We all need that early in the morning

I just spoke with the night receptionist + the bald night duty manager.

explaining - see above post-

we went step by step through the whole process together....

they were both genuine surprised it didn't work,

according to them all criteria were fulfilled:
- email is same as in IHG-account as stayprofile as reservation
- name match with reservation and IHG-account
- reception can see the roomcharge of the F&B at the bar (hotel's owned & operated venue)

* they will have a look at their backoffice end and see what's the problem while i am having breakfast.....
They took a shot of the error message and also mentioned to forward this to the day dutymanager / reception team......

So it seems all infrastructure is in place for the F&B vouchers to be claimed at the Voco The Hague but that IHG's IT is a bit temperamental
so fingers crossed