With Jenkins 1.509.4, Flexible Publish 0.12 and Maven Project Plugin 1.509.4 it works fine.

After you see this error, everything after whatever caused the error in the config.xml disappears, so this is a dangerous problem!!

It reproduced in my environment.

1. Install Flexible Publish and xUnit plugin
2. Create a new Maven 2/3 job.
3. Add the flexible publish post-build action, and configure "Publish xUnit test result report" in it.
4. Save
5. Edit again
6. Save (no need for modification)

I get: Failed to instantiate class org.jenkins_ci.plugins.flexible_publish.ConditionalPublisher
It does not output any stacktraces on browsers nor system logs.

I found it reproduces even free style projects.
I think this happens combined with xUnit plugin (I updated the summary).
Please report if you find same problems with other plugins.

I found wrong codes in Flexible publish handling error messages.
The message "Failed to instantiate class org.jenkins_ci.plugins.flexible_publish.ConditionalPublisher" was wrong,
but should be:

Failed to instantiate: class not found ["hudson.tasks.test.AggregatedTestResultPublisher","hudson.tasks.ArtifactArchiver","hudson.tasks.BuildTrigger","hudson.tasks.junit.JUnitResultArchiver","hudson.tasks.JavadocArchiver","org.jenkinsci.plugins.xunit.XUnitPublisher","hudson.tasks.Fingerprinter","hudson.tasks.Mailer"]

I'll fix this problem (improper error message) in the next release.

The root cause is that, Jenkins passes a broken json data.
I attach that json.
"stapler-class" should be the class name of the selected publisher, but passed a list of all publishers.

I'm not sure why this happens only when combined with xUnit plugin.

Code changed in jenkins
User: ikedam
Path:
src/main/java/org/jenkins_ci/plugins/flexible_publish/ConditionalPublisher.java
http://jenkins-ci.org/commit/flexible-publish-plugin/9145b7c4efeed18c0df027f8e378ce099edda8e4
Log:
JENKINS-21497 Fixed a wrong error message when failed to instantiate a publisher.

Compare: https://github.com/jenkinsci/flexible-publish-plugin/compare/f60940f04492...9145b7c4efee

xunit-plugin generates broken HTML like this:

        <tr>
          <td class="setting-leftspace"> </td>
          <td class="setting-name">Choose your threshold mode</td>
          <td class="setting-main">
           <input name="thresholdMode" value="1" type="radio" checked="true" />
           <label class="attach-previous">Use a number of tests</label>
           <br />
           <input name="thresholdMode" value="2" type="radio" /><label class="attach-previous">Use a percent of tests</label>
          </td>
        </tr>
        <tr class="validation-error-area"><td colspan="2"></td><td></td></tr>
      </tbody></table>
    </td></tr>
  </td></tr>
  <br /><br />  <!-- This is wrong HTML!!!! -->
  <tr><td colspan="3">
    <tr>
      <td></td>
      <td></td>
      <td>

Web browsers converts this HTML like this:

...
<tr class="dropdownList-container">
  <td colspan="2"></td>
  <td colspan="2">
    <br /><br />  <!-- wrong HTML comes here -->
    <table name="publisher" width="100%">
    <tbody>
      <tr class="dropdownList-start rowvg-start">
      <td style="display:none">
         <input name="stapler-class" value="hudson.tasks.test.AggregatedTestResultPublisher" type="hidden">
      </td>
...

This cause dropdownList work wrong as invalid "br" tags prevents it find out "tr.dropdownList-start".

I created a ticket for xunit plugin: JENKINS-21532
This would be fixed in that ticket. Won't fix in this ticket.

Please contact to the maintainer of xunit plugin if you want to hurry that fix.

Fast work! You guys rock!

Fixed in 0.13.
It will be available in a day.
Please try that.