A4j buttons don't fire

Question

I used richfaces in a JSF project. Now I'm trying the same with Seam. But I always get the same JavaScript error:

Message: Object required
Line: 2791
Code: 0
URI: http://localhost:8080/myapp/a4j/g/3_3_1.GA/org/ajax4jsf/framework.pack.js

The s:button tag works

 <s:button id="eMail0" value="sButton" action="#{Mailer.sendMail}" /> 

Both of the following lead to the described JavaScript message in IE8 (Firefox shows no response)

 <a4j:commandButton id="eMail1" value="cmdButton" action="#{Mailer.sendMail}" />

 <a4j:commandLink id="eMail2" action="#{Mailer.sendMail}">
     <f:param name="email" value="param"/>
     <h:outputText value="eMailLink"/>
 </a4j:commandLink>

Any ideas on how I could work around that issue, or is there a known issues with seam 2.2.0?