Error validating xml with dtd

I am trying to validate an xml against a schema ( which is generated
using a valid dtd) using pub.schema:validate in Webmethods 3.6 B2B Server
Build 530. Even though the xml is valid, I get the below listed error
message.

errorMessage: Ambiguous content model in schema - not LL(1)
errorCode: NV-009
path: /TXLife/TXLifeRequest/

The documentation in B2B developers guide[more in attachments] suggests that this could be when we have an invalid child element in the path indicated. But I am sure I dont have any.

Does anyone know what does LL(1) refer to coz I dont have any such element in my dtd.