Workaround (untested):

unarchive mapping: ['' + requiredFile: '.']

In Groovy, you have to escape symbols for map literals keys:

[(requiredFile): 'xxx']

http://groovy-lang.org/groovy-dev-kit.html#_map_literals :

Map keys are strings by default: [a:1] is equivalent to ['a':1]. This can be confusing if you define a variable named a and that you want the value of a to be the key in your map. If this is the case, then you must escape the key by adding parenthesis

That means the workaround of using a GString as a key should not be needed

The above does not work and still generates the exception:

def file = "name-${env.BUILD_NUMBER}"
[(file) : "."] still results in

java.lang.ClassCastException: org.codehaus.groovy.runtime.GStringImpl cannot be cast to java.lang.String
at org.jenkinsci.plugins.workflow.steps.ArtifactUnarchiverStepExecution.run(ArtifactUnarchiverStepExecution.java:44)
at org.jenkinsci.plugins.workflow.steps.ArtifactUnarchiverStepExecution.run(ArtifactUnarchiverStepExecution.java:20)
at org.jenkinsci.plugins.workflow.steps.AbstractSynchronousNonBlockingStepExecution$1$1.call(AbstractSynchronousNonBlockingStepExecution.java:49)
at hudson.security.ACL.impersonate(ACL.java:213)
at org.jenkinsci.plugins.workflow.steps.AbstractSynchronousNonBlockingStepExecution$1.run(AbstractSynchronousNonBlockingStepExecution.java:47)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

I found this thread because I was running into the exact same error. I was able to get it working by adding .toString() along with the () in the mapping.

So in this case it would be:
def file = "name-${env.BUILD_NUMBER}".toString()
[(file) : "."]

HTH

This works in workflow-cps 2.39 (with the caveat that, as elsewhere in Groovy, the key has to either be (var) or "${var}"), and I've got a PR up at https://github.com/jenkinsci/workflow-cps-plugin/pull/161 adding a test for it.

Probably same as JENKINS-43934.

Code changed in jenkins
User: Andrew Bayer
Path:
src/test/java/org/jenkinsci/plugins/workflow/cps/CpsFlowDefinition2Test.java
http://jenkins-ci.org/commit/workflow-cps-plugin/329f900d776c96f4f91576bb7bda3bbde31a26e1
Log:
JENKINS-27916 Test verifying use of GStrings in map keys