If I uncomment the sessionIdAlphabet element in $EAP_HOME/server/$PROFILE/deployers/jbossweb-deployer/META-INF/war-deployers-jboss-beans.xml as following:
<!-- The SessionIdAlphabet is the set of characters used to create a session Id
It must be made up of exactly 65 unique characters-->
<property name="sessionIdAlphabet">ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+-_</property>

I'm getting errors during the server startup:
java.lang.RuntimeException: Error configuring property: sessionIdAlphabet for WarDeployer
...
Caused by: java.lang.NoClassDefFoundError: org/jboss/web/tomcat/service/session/SessionIDGenerator
at org.jboss.web.tomcat.service.deployers.TomcatDeployer.setSessionIdAlphabet(TomcatDeployer.java:291)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
...

If I copy jboss-web-service.jar from /deploy/jbossweb.sar to /deployers/jbossweb.deployer and the server is starting up without the error.