If cross boarder payments are being rejected, this is likely because the cross border payment limit has been exceeded.
By default, the limit is set to 0, so the PSU will need to change the limit in their Commerzbank online banking account.
Once the limit has been increased, they can retry the payment and it should be successful.
What is classified as a cross boarder payment?
All payments which have a non-German IBAN for the creditor is considered as cross border.