Every time when I want to discard old data got:

(even if there is a one error showed)

java.lang.NullPointerException
at java.util.TreeMap.putAll(TreeMap.java:313)
at java.util.TreeMap.<init>(TreeMap.java:185)
at hudson.model.Fingerprint.toString(Fingerprint.java:1415)
at java.lang.String.valueOf(String.java:2994)
at java.lang.StringBuilder.append(StringBuilder.java:131)
at hudson.diagnosis.OldDataMonitor.saveAndRemoveEntries(OldDataMonitor.java:365)
at hudson.diagnosis.OldDataMonitor.doDiscard(OldDataMonitor.java:335)
at java.lang.invoke.MethodHandle.invokeWithArguments(MethodHandle.java:627)
at org.kohsuke.stapler.Function$MethodFunction.invoke(Function.java:343)
at org.kohsuke.stapler.interceptor.RequirePOST$Processor.invoke(RequirePOST.java:77)
at org.kohsuke.stapler.PreInvokeInterceptedFunction.invoke(PreInvokeInterceptedFunction.java:26)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:184)
at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:117)
at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:129)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:58)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:734)
Caused: javax.servlet.ServletException