Why does custom date format 'EEE' show day-of-week index?

Why does custom date format 'EEE' show day-of-week index?

I have used the custom date format 'EEE' to display the day of week in
short form. i.e. Mo, Di, Mi (in German). In my development environment
(eclipse) it works as desired. However on both the Test and Production
platform I get the day-of-week index - 1, 2, 3, ...
Other Date and Numerical formats in the report are working fine and are
appropriate for the German Locale.
Any ideas what might be causing this?
<cell id="2347">
<property name="borderRightColor">#C0C0C0</property>
<property name="borderRightStyle">solid</property>
<property name="borderRightWidth">thin</property>
<data id="2381">
<structure name="dateTimeFormat">
<property name="category">Custom</property>
<property name="pattern">EEE</property>
</structure>
<property name="textAlign">center</property>
<property name="resultSetColumn">Date</property>
</data>
<data id="2388">
<structure name="dateTimeFormat">
<property name="category">Custom</property>
<property name="pattern">d. MMM</property>
</structure>
<property name="textAlign">center</property>
<property name="resultSetColumn">Date</property>
</data>
</cell>
What I expect is
Mi
11. Sep
what I get is
4
11. Sep