Just got more feedback:
"for a type "float" it's not possible to have an empty value, set to 0 instead
<EstimatedHours></EstimatedHours>
put:
<EstimatedHours>0</EstimatedHours>"
Not sure if this is according to standard WSDL or application specific on the other's side. I can put all of that on a separate page for other developers what I had to keep in mind when dealing with WSDL in general and TWiST in particular, as I am sure I will miss a couple of more things.