Redmine

I just tried the following input, which returns

Started PUT "/issues/3273.xml?key=asdf" for 23233 at 2014-08-28 10:23:28 +0200
Processing by IssuesController#update as XML
  Parameters: {"issue"=>{"id"=>"3273", "subject"=>"ch.elexis.base.ch.legacy.feature.feature.group", 
"description"=>nil, "start_date"=>"2014-08-20T00:00:00+02:00", "project_id"=>"0", "tracker_id"=>"0", 
"custom_fields"=>[{"id"=>"2", "name"=>"Abostatus", "value"=>"EXPIRED"}], "project"=>{"id"=>"3", 
"name"=>"Praxis Dr. Mustermann"}, "tracker"=>{"id"=>"4", "name"=>"Service"}}, "key"=>"asdf", "id"=>"3273"}
  Current user: mustermann (id=5)
Completed 200 OK in 27.9ms (Views: 0.4ms | ActiveRecord: 3.2ms

yet the value of the custom field in the issue is not changed, here the input

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<issue>
 <id>3273</id>
 <subject>ch.elexis.base.ch.legacy.feature.feature.group</subject>
 <description/>
 <start_date>2014-08-20T00:00:00+02:00</start_date>
 <project_id>0</project_id>
 <tracker_id>0</tracker_id>
 <custom_fields type="array">
   <custom_field id="2" name="Abostatus">
     <value>EXPIRED</value>
   </custom_field> 
 </custom_fields>
 <project id="3" name="Praxis Dr. Mustermann"/>
 <tracker id="4" name="Service"/>
</issue>

The respective field is defined as list with possible values.