Error in my log:
Caused by: java.lang.NullPointerException
at hudson.scm.CvsTagsParamDefinition.getCvsClient(CvsTagsParamDefinition.java:223)
at hudson.scm.CvsTagsParamDefinition.getSymbolicNames(CvsTagsParamDefinition.java:165)

Probably problem is in this code:

envVars = Computer.currentComputer().getEnvironment();

currentComputer is null like in this issue #JENKINS-5237.

Added a potential fix in this pull request: https://github.com/jenkinsci/cvs-plugin/pull/43

Code changed in jenkins
User: Fred G
Path:
src/main/java/hudson/scm/CvsTagsParamDefinition.java
http://jenkins-ci.org/commit/cvs-plugin/1b6a75dd59a6a61cc7570746e488342d8096d3ca
Log:
[FIXED JENKINS-26345] "CVS Symbolic Name parameter" not working any more

Code changed in jenkins
User: Fred G
Path:
src/main/java/hudson/scm/CvsTagsParamDefinition.java
http://jenkins-ci.org/commit/cvs-plugin/25e0d5f1878578358f21ca40310ef1efa81caee3
Log:
Merge pull request #43 from fredg02/JENKINS-26345

[FIXED JENKINS-26345] "CVS Symbolic Name parameter" not working any more

Compare: https://github.com/jenkinsci/cvs-plugin/compare/69b0faa00c1c...25e0d5f18785

I installed latest generic up-to-date version of Jenkins (LTS  version 2.46.3) on SLES12 server, installed latest CVS-Plugin v.2.13, created and ran test job.

The plugin still does not work. It didn't show list of CVS tags and I could see error in log:

Caught exception evaluating: it.symbolicNames in /jenkins/job/test_maven_1/build. Reason: java.lang.reflect.InvocationTargetException
java.lang.NullPointerException
    at hudson.scm.CvsTagsParamDefinition.getCvsClient(CvsTagsParamDefinition.java:223)
    at hudson.scm.CvsTagsParamDefinition.getSymbolicNames(CvsTagsParamDefinition.java:165)
Caused: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

...

The plugin still does not work. It didn't show list of CVS tags and I could see error in log:

The changes that fix the error are not included in version 2.13. They will be included in the next release of the CVS-Plugin.

If you don't want to wait for the next release, you will need to compile the plugin from source yourself.