Hi,
thanks for your question.
Indeed there is a known issue related to the offline activation method which you are currently facing.
The problem is that instead of Activation XML response type:
<ResponseType>ACTIVATION</ResponseType>
some responses are different type and cause an error "Invalid Offline Response"
<ResponseType>CLIENT_CONFIG</ResponseType>
Unfortunately currently the only workaround is to use online activation instead.
Seems that online activation works fine on your side.
Stan