In the mean time I figured out what the problem is. It seems that when I deactivate the Mollie payment plugin and simply re-activate it (and doing nothing in between) the problem disappeared.
Thank you for your answer.
Customer login:
https://staging13.alldetect.nl/wp-admin
Name: Woo
Passw: c@EmpOfJOx0MuGGnAR5n)h*u
I can also share admin login if you wish. How can I do that without showing it to the whole world?
