[31]File Signature not Valid

Hello dear community,

I am relatively new to oftp2, but we need to connect a customer.
The customer can send us his data without any problems.

Unfortunately, we cannot transfer any data to the customer, we only get an error message:
Negative receipt acknowledgement (NERP) received [Customer- us], the reason code is [31] ("File signature not valid")

We have checked all the necessary certificates and also the hash several times.
The customer also claims that he has set everything correctly.

google has not been much help so far, maybe I can find someone here who could help me.

Thank you and have a nice day