BTW. I know this is happens only to my app. Seam examples work fine even if exploded.
Solved ! The problem was that my EJB module had jboss-seam.jar in Class-Path attribute in MANIFEST.MF. That is no problem when I deploy application as packaged archive, but when I deploy it as exploded directory jboss-seam.jar gets loaded twice or something like that.
Could anybody explain it a bit?
It means that JSF finds Seam twice on the classpath. We don't recommend using MANIFEST.MF Class-Path with Seam, but people are complaining about that, so please file a JIRA issue, so we can at least document it better.