It seems to work now, but I am not sure why
The only thing that I changed is the number of mailings.
Initially I had two as outlined in step 2 of the manual, Mailing no. 1 for questionnaire A2 (that I was actually not using) and Mailing no. 2 for the invitation to the second wave.
Then I simply deleted Mailing no. 1, and now it works correctly
Best wishes