It seems to be a bug in Mojarra (we have been using the version 2.1.6), that EL lost the namespace "rich". A working workaround was to declare the namespace in the a4j:ajax tag for each use:
<myLib:commandLink value="Show">
<a4j:ajax render="@form" execute="@form" xmlns:rich="http://richfaces.org/rich"
oncomplete="#{rich:component('myEditPopup')}.show()"/>
</myLib:commandLink>
After updating Mojarra to 2.1.26 the problem is gone and no needs for this workaround.