it sometimes happens also if root tag is in default namespace and someother tag is in default namespace too - then xmlns:xmlns is attached to this tag (I saw this on validates or transforms tags)

Just to confirm - adding the default namespace - as you show in the switchyard2.xml file you attached - does this avoid the issue in 100% of the cases?

no, it can happen on other element with default namespace inside of document, I haven't found a cause yet.

It's due to the bpel-component dragging in saxonhe and xalan into build classpath.  I will reply with more detail and another workaround in a sec.

Before I get into details, I want to reiterate that this only occurs with projects which include a dependency on switchyard-component-bpel (i.e. they use a BPEL service).

I can eliminate this error with the following exclusions:

<dependency>
  <groupId>org.switchyard.components</groupId>
  <artifactId>switchyard-component-bpel</artifactId>
  <exclusions>
    <exclusion>
      <groupId>xalan</groupId>
      <artifactId>xalan</artifactId>
    </exclusion>
    <exclusion>
      <groupId>net.sourceforge.saxon</groupId>
      <artifactId>saxonhe</artifactId>
    </exclusion>
  </exclusions>
</dependency>

Unfortunately, this only works because the reproducer project doesn't actually use the BPEL implementation.  Using the above exclusions on a project which dose use BPEL results in an error.  Interestingly enough, the bpel-service quickstarts do not exhibit this issue an they do not use the default (xmlns="...") namespace.  There must be a combination or ordering of dependencies for BPEL projects which makes this possible, but I haven't been able to isolate it yet.  For the time being, I would recommend the workaround of assigning the default namespace explicitly in projects which use BPEL.

*** Bug 1047938 has been marked as a duplicate of this bug. ***