It seems like the selenium plugin is already installed and running. Could you try removing the plugin and installing again?

I have never had this happen when installing the plugin, so honestly I do not know. Perhaps you could try to find out what process is using the file C:\Users\tdjenkins\.jenkins\plugins\selenium\WEB-INF\lib\commons-lang3-3.4.jar and kill that process?

The process it was attached to is Java. I tried rebooting the machine and reinstalling the plugin but it didn't work.

I installed a fresh copy of Jenkins 2.2 locally and installed the Selenium plugin and it worked. I then copied that Selenium plugin folder and pasted it to the plugin folder of the deployed Jenkins instance and restarted Jenkins. This was the error:

May 10, 2016 1:27:27 PM jenkins.InitReactorRunner$1 onTaskFailed
SEVERE: Failed Loading plugin selenium
java.io.IOException: Failed to initialize
at hudson.ClassicPluginStrategy.load(ClassicPluginStrategy.java:519)
at hudson.PluginManager$2$1$1.run(PluginManager.java:415)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:169)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:282)
at jenkins.model.Jenkins$7.runTask(Jenkins.java:996)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:210)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:117)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: java.io.IOException: Unable to read C:\Users\tdjenkins\.jenkins\selenium.xml
at hudson.XmlFile.unmarshal(XmlFile.java:165)
at hudson.Plugin.load(Plugin.java:264)
at hudson.plugins.selenium.PluginImpl.start(PluginImpl.java:141)
at hudson.ClassicPluginStrategy.startPlugin(ClassicPluginStrategy.java:527)
at hudson.ClassicPluginStrategy.load(ClassicPluginStrategy.java:516)
... 9 more
Caused by: com.thoughtworks.xstream.converters.ConversionException: null : null
---- Debugging information ----
cause-exception : java.lang.NullPointerException
cause-message : null
class : hudson.plugins.selenium.PluginImpl
required-type : hudson.plugins.selenium.PluginImpl
converter-type : hudson.util.RobustReflectionConverter
path : /hudson.plugins.selenium.PluginImpl
line number : 83
version : not available
-------------------------------
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:79)
at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:65)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:66)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:50)
at com.thoughtworks.xstream.core.TreeUnmarshaller.start(TreeUnmarshaller.java:134)
at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.unmarshal(AbstractTreeMarshallingStrategy.java:32)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1189)
at hudson.util.XStream2.unmarshal(XStream2.java:114)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1173)
at hudson.XmlFile.unmarshal(XmlFile.java:163)
... 13 more
Caused by: java.lang.NullPointerException
at hudson.diagnosis.OldDataMonitor.report(OldDataMonitor.java:219)
at hudson.util.RobustReflectionConverter.doUnmarshal(RobustReflectionConverter.java:330)
at hudson.util.RobustReflectionConverter.unmarshal(RobustReflectionConverter.java:229)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:72)
... 22 more

Replacing the existing plugin in this way does not work because there was a configuration change between the last and the current version which makes the old configuration file invalid. That is what is causing the crash now as is shown by this line: Caused by: java.io.IOException: Unable to read C:\Users\tdjenkins\.jenkins\selenium.xml.
If the plugin update had worked, jenkins would have taken care of this, but because of manually replacing the plugin, the configuration file is not converted to the new format and jenkins fails on loading the configuration.
If you remove the C:\Users\tdjenkins\.jenkins\selenium.xml file the plugin should load fine, but this does mean having to setup the plugin configuration again.

Thanks mobrockers it worked!

Issue has been fixed for user but might be an unidentified bug underneath, not sure.

Some backstory: Issue arose when we upgraded from Jenkins 1.6.* to 2.0. The Selenium plugin failed to install and the only error was logged in jenkins.err. The original error printed was: Caused by: java.io.IOException: Unable to read C:\Users\tdjenkins\.jenkins\selenium.xml