"Loop / Attributes not found"

Hello everybody!

I try to access attributes via the Loop Attributes operator.
My dataset consists of only two attributes, but many examples.

Within the loop I want to do a FFT (from the Series extension) on every series I built from the examples.
So in the Data to Series operator (located within the loop), I access every attribute by the macro's name (here: %{loop_attributes}). In the metadata, "att1" and "att2" as attribute-names are visible. But the Data to Series operator complains about not being able to find "att2".

Can someone please have a look at my process (see below) and hint me to a solution?

Thank you a lot for any advice!

Yours
Benjamin

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<process version="5.2.006">
  <context>
    <input/>
    <output/>
    <macros/>
  </context>
  <operator activated="true" class="process" compatibility="5.2.006" expanded="true" name="Process">
    <process expanded="true" height="612" width="1272">
      <operator activated="true" class="retrieve" compatibility="5.2.006" expanded="true" height="60" name="Retrieve (2)" width="90" x="45" y="30">
        <parameter key="repository_entry" value="../daten/zwei_reihen_v"/>
      </operator>
      <operator activated="true" class="loop_attributes" compatibility="5.2.006" expanded="true" height="60" name="Loop Attributes" width="90" x="447" y="120">
        <parameter key="attribute_filter_type" value="subset"/>
        <parameter key="attributes" value="att1|att2|"/>
        <process expanded="true" height="630" width="1284">
          <operator activated="true" class="series:data_to_series" compatibility="5.1.002" expanded="true" height="60" name="Data To Series" width="90" x="179" y="30">
            <parameter key="series_attribute" value="%{loop_attribute}"/>
          </operator>
          <operator activated="true" class="series:fast_fourier_transformation" compatibility="5.1.002" expanded="true" height="60" name="Fast Fourier Transformation" width="90" x="447" y="30"/>
          <operator activated="true" class="series:series_to_data" compatibility="5.1.002" expanded="true" height="60" name="Series To Data" width="90" x="715" y="30"/>
          <connect from_port="example set" to_op="Data To Series" to_port="example set"/>
          <connect from_op="Data To Series" from_port="series" to_op="Fast Fourier Transformation" to_port="series"/>
          <connect from_op="Fast Fourier Transformation" from_port="series" to_op="Series To Data" to_port="series"/>
          <connect from_op="Series To Data" from_port="example set" to_port="example set"/>
          <portSpacing port="source_example set" spacing="0"/>
          <portSpacing port="sink_example set" spacing="0"/>
        </process>
      </operator>
      <connect from_op="Retrieve (2)" from_port="output" to_op="Loop Attributes" to_port="example set"/>
      <connect from_op="Loop Attributes" from_port="example set" to_port="result 1"/>
      <portSpacing port="source_input 1" spacing="0"/>
      <portSpacing port="sink_result 1" spacing="0"/>
      <portSpacing port="sink_result 2" spacing="0"/>
    </process>
  </operator>
</process>