Dimension mismatch, List expected


I’ve created a Web Service from an existing WSDL in developer. The parameter of the Servicemethod is a complex type with a one-to-many associtaion (optional, unbound).
The implementation of the service just logs one parameter using the pub.log.debug service. When I try to call the service using soapUI I get a validation error from IS if the SOAP-Message contains a node of the one-to-many associtaion:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
         <faultstring>[ISS.0088.9112] An Exception was thrown in the server</faultstring>
         <detail xmlns:webM="http://www.webMethods.com/2001/10/soap/encoding">
               <webM:message xml:lang="">[ISC.0049.9005] Input validation for service 'services.MBServices_.services:Store' failed: '/ns1:Message/ns1:message/ns1:attachments VV-003 [ISC.0082.9027] Dimension mismatch, List expected'</webM:message>

Validating the xml sent in soapUI against the schema yields no error. Also looking at the message using tcpMon shows a valid SOAP-message.

i’ve read the post of similar problems, that were solved setting makeArrays=true and specify arrays=Item. However i can’t see how i can apply this solution to the problem mentioned. Especially since the document model has been created by developer itself.
When I omit the node containg the one-to-many relationship or disable validation at all, the validation succeeds.

Can anybody help me, were to start looking ?

Thanks in advance.