Biztalk Rule Composer - Xpath Selector for Multiple Records

[dime]

I have a xml file containing multiple records and inside each single record I
want to set certain attributes on base of other attributes'value (attributes
are on different fields/hierarchy inside the record) with help of the
business rule composer. This is the initial sample xml file:

<ns0:import xmlns:ns0="http://school.org">
<content kind="Student">
<identification accountName="tim.taylor" />
<changes>
<change property="phone" oldValue="123" newValue="456" />
/>
</changes>
</content>
<content kind="professor">
<identification accountName="1234" />
<changes>
<change property="mail" oldValue="old" newValue="new" />
</content>
</changes>
</ns0:import>

For example if <content> attribute @kind equals "professor" set the
@property value to "ignore" on the <change> field.

I followed the instructions on these links:
http://msdn.microsoft.com/en-us/library/aa561822(BTS.10).aspx
http://www.mofeel.net/20-microsoft-public-biztalk-general/3423.aspx

But I always get the following error:
Field "/*[local-name()='changes' and
namespace-uri()='']/*[local-name()='change' and
namespace-uri()='']/@*[local-name()='property' and namespace-uri()='']" does
not exist in XML document "Messages", selector "/tn:import/content" when I
test my policy with the given sample xml.

The xpath Selector and xpath Field are as given in the error message above.

I checked the complete xpath path of the action statement with an external
tool and it points correctly to the property attribute.

Any thoughts on this issue are welcome.
greets
dime