In hudson log, lots of such messages recently appeared (i.e. after having upgraded subversion plugin from 1.17 to 1.20):

hudson.scm.PerJobCredentialStore <init>
INFO: Failed to retrieve Subversion credentials from /var/lib/hudson/jobs/JOB_NAME/subversion.credentials
hudson.util.IOException2: Unable to read /var/lib/hudson/jobs/JOB_NAME/subversion.credentials
at hudson.XmlFile.unmarshal(XmlFile.java:152)
at hudson.scm.PerJobCredentialStore.<init>(PerJobCredentialStore.java:38)
at hudson.scm.SubversionSCM$DescriptorImpl.createAuthenticationProvider(SubversionSCM.java:1669)
at hudson.scm.SubversionChangeLogBuilder.createAuthenticationProvider(SubversionChangeLogBuilder.java:122)
at hudson.scm.SubversionChangeLogBuilder.getUrlForPath(SubversionChangeLogBuilder.java:118)
at hudson.scm.SubversionChangeLogBuilder.run(SubversionChangeLogBuilder.java:103)
at hudson.scm.SubversionSCM.calcChangeLog(SubversionSCM.java:440)
at hudson.scm.SubversionSCM.checkout(SubversionSCM.java:557)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1119)
at hudson.model.AbstractBuild$AbstractRunner.checkout(AbstractBuild.java:479)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:411)
at hudson.model.Run.run(Run.java:1324)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:139)
Caused by: com.thoughtworks.xstream.converters.ConversionException: No InvocationHandler specified for dynamic proxy
---- Debugging information ----
class : com.thoughtworks.xstream.mapper.DynamicProxyMapper$DynamicProxy
required-type : com.thoughtworks.xstream.mapper.DynamicProxyMapper$DynamicProxy
path : /hudson.scm.PerJobCredentialStore
line number : 13
-------------------------------
at com.thoughtworks.xstream.converters.extended.DynamicProxyConverter.unmarshal(DynamicProxyConverter.java:92)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:82)
at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:63)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:76)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:60)
at com.thoughtworks.xstream.core.TreeUnmarshaller.start(TreeUnmarshaller.java:137)
at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.unmarshal(AbstractTreeMarshallingStrategy.java:33)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:926)
at hudson.util.XStream2.unmarshal(XStream2.java:80)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:912)
at hudson.XmlFile.unmarshal(XmlFile.java:148)
... 14 more

This only occurs when the job's subversion.credentials looks like this:

<?xml version='1.0' encoding='UTF-8'?>
<hudson.scm.PerJobCredentialStore resolves-to="dynamic-proxy">
  <h class="hudson.remoting.RemoteInvocationHandler" serialization="custom">
    <hudson.remoting.RemoteInvocationHandler>
      <default>
        <autoUnexportByCaller>false</autoUnexportByCaller>
        <goingHome>false</goingHome>
        <oid>264</oid>
        <userProxy>true</userProxy>
      </default>
    </hudson.remoting.RemoteInvocationHandler>
  </h>
</hudson.scm.PerJobCredentialStore>

note: deserialization succeeds when the job's subversion.credentials looks like that:

<?xml version='1.0' encoding='UTF-8'?>
<hudson.scm.PerJobCredentialStore>
  <credentials class="hashtable">
    <entry>
      <string>REALM</string>
      <hudson.scm.SubversionSCM_-DescriptorImpl_-PasswordCredential>
        <userName>USERNAME</userName>
        <password>PASSWORD</password>
      </hudson.scm.SubversionSCM_-DescriptorImpl_-PasswordCredential>
    </entry>
  </credentials>
</hudson.scm.PerJobCredentialStore>

Let me know if something else is required.