Confirmation of vAT declaration for quarter 1

Dear

I submitted my vAT declaration for quarter 1 some time ago. I did this on time via Intervat. Everything went smoothly and is okay. I have already checked this. However, I am unable to upload my file in Dexter, which means I keep getting a message saying that I have not done this yet. Even though I did do it on Intervat. I have already checked with other Dexter users and they have also tried, but I keep getting the message that I am not entering the correct file.

Can you help me further?

Thanks in advance!