Java stored procedure or user-defined function could not call Java method

After I created a Java external stored procedure in DB2 as in my previous question, the call

call user1.send_mail(    P_TO          => 'me@domain.com'   ,P_CC          => NULL   ,P_BCC         => NULL   ,P_FROM        => 'me@domain.com'   ,P_SUBJECT     => 'db2 java subject'   ,P_TEXT_MSG    => 'db2 java test'   ,P_HTML_MSG    => null   ,P_ATTACH_NAME => null   ,P_ATTACH_MIME => null   ,P_ATTACH_BLOB => null   ,P_SMTP_HOST   => 'domain.com'   ,P_SMTP_PORT   => 465   ,P_USER_NAME   => 'me@domain.com'   ,P_PASSWORD    => 'secret'); 

fails it due to the following error:

Java stored procedure or user-defined function "USER1.SEND_MAIL", specific name "SQL201208113215436" could not call Java method "send_mail", signature "(Ljava/lang/String".. SQLCODE=-4306, SQLSTATE=42724, DRIVER=3.68.61

I made sure that all parameters that have no default values receive non-NULL values. What else is necessary for this to work?