A more detailed description of this issue, along with an example, can be found in the Pabot repository: mkorpela/pabot#670

Since Pabot uses Rebot internally to parse results, and Rebot seems to generate a JUnit XML with <testsuite> as the root element instead of <testsuites> when there are two or more output.xml files, this small fix should be made in Rebot rather than in Pabot.