I did tests on our lab.
Developers told me that if the mollie payment fails for any reasons an error message simply telling "Payment Error" should be add in the system log.
Can you check in the system log if you see any "Payment Error" message?
Another thing, if the payment at mollie side goes ok, and the user is redirected back to the welcome portal to terminate registration/recharge, HSNM send a request to mollie asking the state of the payment. It should be interesting to know if mollie answered to this request.