How could it happen that the interest rate for the loan usage had been zeroized

The interest rate for the funds usage could have only been zeroized if the lender had made an attempt to perform an early loan repayment. According to paragraph 5.1 of The Rules interest charges are cancelled in this case.

5.1 The Lender has aright to demand the Loan repayment at any time. In this case the Debtor repays only the principal and the Lender refuses to cancel the interest charges. If the interest charges were partially paid before they are not calculated towards the principal amount during early termination.