Thanks!When it will be released 0.21?

Adi, what I did in the mean time was to generate nunit v2 results, which don't seem to cause this effect.

I installed now the 0.21 version of the NUnit plugin and tested it with the attached TestResults4.xml.It failed like before with 0.20 version of the Nunit plugin:
10:53:39 Run condition [Current build status] enabling perform for step [[Publish xUnit test result report]]10:53:39 [xUnit] [INFO] - Starting to record.10:53:39 [xUnit] [INFO] - Processing NUnit-Version N/A (default)10:53:39 [xUnit] [INFO] - [NUnit-Version N/A (default)] - 1 test report file(s) were found with the pattern 'UT/.xml' relative to 'C:\Program Files (x86)\Jenkins\jobs\Publish Nunit Results1\workspace' for the testing framework 'NUnit-Version N/A (default)'.*10:53:39 [xUnit] [ERROR] - The converted file for the result file 'C:\Program Files (x86)\Jenkins\jobs\Publish Nunit Results1\workspace\UT\TestResults4.xml' (during conversion process for the metric 'NUnit') is not valid. The report file has been skipped.10:53:39 [xUnit] [ERROR] - Content is not allowed in prolog.10:53:39 [xUnit] [ERROR] - Content is not allowed in prolog.10:53:39 [xUnit] [ERROR] - There is at least one problem. Check the Jenkins system log for more information. (if you don't have configured yet the system log before, you have to rebuild).10:53:40 [xUnit] [ERROR] - The plugin hasn't been performed correctly: Problem on deletion*10:53:40* Build step 'Flexible publish' changed build result to FAILURE

The error above appeared when I publish the results through Xunit plugin and choosing NUnit.When I use directly the Nunit plugin -Publish Nunit Test results report -it works ok.I suppose you cannot do anything to fix this plugin https://wiki.jenkins.io/display/JENKINS/xUnit+Plugin?

No, they use their own NUnit translation. They don't depend on the NUnit plugin to do the translation from NUnit to JUnit.