Hi Christian,
I think, your system does not support nested method call in a single statement. So, go for individual method call.
Please try the below code:
DATA lv_uri TYPE string.
DATA lo_part TYPE REF TO cl_openxml_part.
DATA lo_parts LIKE wd_this->mo_parts.
DATA lo_part2 LIKE lo_part.
DATA lo_uri TYPE REF TO cl_openxml_parturi.
lo_part = wd_this->mo_parts->get_part( 2 ).
lo_parts = lo_part->get_parts( ).
lo_part2 = lo_parts->get_part( iv_xml_index ).
lo_uri = lo_part2->get_uri( ).
lv_uri = lo_uri->get_uri( ).
Hope this helps you.
Regards,
Rama