Customer with vAT number does not appear in my customer list in Dexxter

Are you sure you entered the vAT number correctly? If there is a typo and the number does not exist, it will not be included in the customer list. The customer list is a regular XML document. You can manually add a customer there (open it with a text editor or Visual Code, not with a word processor or spreadsheet). You must ensure that the structure is retained. See below:

<?xml version="1.0" encoding="ISO-8859-1"?></

<ns2:ClientListingConsignment _blank" href="http://www.minfin.fgov.be/InputCommon">http://www.minfin.fgov.be/InputCommon" xmlns_ns2="http://www.minfin.fgov.be/ClientListingConsignment" ClientListingsNbr="1">

<ns2:ClientListing VATAmountSum="0.00" TurnOverSum="0.00" ClientsNbr="1" SequenceNumber="1">

<Street>street</Street>

<PostalCode>0000</PostalCode>

<City>city</City>

<CountryCode>BE</CountryCode>

<EmailAddress>email address</EmailAddress>

</ns2:Declarant>

<ns2:Period>2024</ns2:Period>

<ns2:TurnOver>0.00</ns2:TurnOver>

<ns2:Client SequenceNumber="1">

<ns2:CompanyVATNumber issuedBy="BE">0000000000</ns2:CompanyVATNumber>

<ns2:Turnover>0.00</ns2:Turnover>

<ns2:VATAmount>0.00</ns2:VATAmount>

</ns2:Client>

</ns2:ClientListing>

</ns2:ClientListingConsignment>

The bold text in the first part has always been correctly filled in by Dexxter (I have set the amounts to 0.00 here and replaced my own company details with 0000000000 for the vAT number and company name, street, 0000 (for postal code), city, and email address.
The customers state under the tag <ns2:TurnOver> and the end tag </ns:ClientListing>. For each customer, there will be a tag <ns2Client> </ns2:Client>. So you just need to add a new tag for the missing customers after the last <ns2Client> </ns2:Client> tag and before the end tag </ns2:ClientListing>:

<ns2:Client SequenceNumber="1">

<ns2:CompanyVATNumber issuedBy="BE">0000000000</ns2:CompanyVATNumber>

<ns2:Turnover>0.00</ns2:Turnover>

<ns2:VATAmount>0.00</ns2:VATAmount>

</ns2:Client>In that tag, you then perform a manual change of the SequenceNumber 1 to the number of the last + 1, 0000000000 to the customer's vAT number, and 0.00 in the tag <ns2:TurnOver> with the total amount excl. VAT invoiced to the customer and 0.00 in the tag <ns2:VATAmount> with the vAT amount applicable to that customer.

Please note that you must enter numbers with a . (and not a ,) as the decimal point and save the file as .xml (not as a text file). If you use a standard text editor such as Notepad, it will save as .xml.txt. In that case, you must edit the extension to .xml in Explorer and confirm.

Thank you for your response. I have double-checked: the vAT number is correct. I will add it manually. I am just wondering why it is not being picked up by Dexxter...

It is best to ask that question via the support button to the Dexxter team itself.