"Cannot modify entity_table"

When trying to compose or send a mailing to 4 or 7.000 people I get the API exception "Cannot modify entity_table". When manipulating the function throwing the error, I get:

apiRequest:Array (
[id] => 11
[version] => 3
[params] => Array (
[id] => 4089
[name] => Zuwendungen_2013_169.pdf
[mime_type] => application/pdf
[description] =>
[upload_date] => 2014-01-24 00:00:00
[entity_table] => civicrm_value_bescheinigungen_6
[entity_id] => 169
[url] => DOMAIN/index.php?q=civicrm/file&reset=1&id=4089&eid=169
[path] => PATH/sites/default/files/civicrm/custom//Zuwendungen_2013_169_be31027b79324d3c11fd468ecdc7abc9.pdf
[check_permissions] => 1
[version] => 3
)
[extra] =>
[fields] =>
[entity] => Attachment
[action] => create
[function] => civicrm_api3_attachment_create
[is_generic] =>
)
fileId:4089
entityTable:civicrm_contact
entityId:169

Contact 169 is none of the recipients nor did I attache a file - especially not this donation receipt.

This is the first mailing in our system after upgrading CiviCRM.