XMlLport Error - Multiple root elements!

add a root Tag (SourceType = Text) at the top, so you get a correct xml-structure:

<root>
<customer field1 field2 field3 />
<customer field1 field2 field3 />
</root>

Hmm, this works usually fine for me...

Maybe if you filter the Customer Records when calling the XML Port helps:

Customer.SETRANGE("No.");
NewFile.CREATE(Filename);
NewFile.CREATEOUTSTREAM(ExportStream);
XMLPORT.EXPORT(_yourXMLPort,ExportStream,Customer);

I think your XMLPort is set up fine, except for one thing. XMLPorts do NOT know how to update existing records, they only know how to create new records. If you want to also update existing records, you need to make your table elements temporary, and in your OnPostXMLPort you loop through the temp table and create/update your records.

What I did for an Item XMLPort is call my root element TempInboundItem (by the way, you can have different tag names and variable names, so be careful you keep them apart in your code) and import the XML document into the temp table. Because it's a temp table, everything is new, so everything gets imported. Then, in the OnPostXMLPort trigger I do something like this (the actual code does a lot more, but this is the basic idea):

IF TempInboundItem.FIND THEN BEGIN
  REPEAT
    // locItem = local rec variable
    IF NOT locItem.GET(TempInboundItem."No.") THEN BEGIN
      locItem.TRANSFERFIELDS(TempInboundItem);
      locItem.INSERT;
    END ELSE BEGIN
      locItem.TRANSFERFIELDS(TempInboundItem);
      locItem.MODIFY;
    END;
  UNTIL TempInboundItem.NEXT = 0;
END;

You could also do IF NOT INSERT THEN MODIFY, but that is less efficient code, and this way you can expand on each scenario.

Okay, I'll try to make a walkthrough:

This is what tht XML-Port should look like (I did not change any Properties, root has identation 0, customer has identation 1 and the other fields have identation 2):

TagName	TagType	SourceType	DataSource
root	Element	Text	<root>
customer	Element	Table	<Customer>(Customer)
No	Attribute	Field	Customer::No.
name	Attribute	Field	Customer::Name
adress	Attribute	Field	Customer::Address

If you want to export all the Customers, use:
Customer.SETFILTER("No."); (you should not use a FIND afterwards).

If this does not work, I can send you the objects.

rCustomer.SETFILTER("No.", '') sets a filter on where No equals '', that does not clear the filter. If you want to clear the filter on the No field, you need to do rCustomer.SETFILTER("No.")