I tried to narrow this bug down, but there isn't much information. We just upgraded to all newest plugins, but unfortunately we upgraded a lot at once, so no idea which one.

This is spamming out logs every few seconds:

00:19:42.695 Cannot contact kubernetes-ef39fe82c8a541be84bd780e4d7c1ddb-ce4d47fc96bfc: java.io.IOException: corrupted content in /home/jenkins/workspace/Robusta_robusta_develop-6EPNQBJK5BYEXOJV6L45MMZZGUIP7WO4Y6EGRUYNFFMRC7B2GL3A@tmp/durable-96fa79b7/pid: java.lang.NumberFormatException: For input string: ""
00:19:57.758 Cannot contact kubernetes-ef39fe82c8a541be84bd780e4d7c1ddb-ce4d47fc96bfc: java.io.IOException: corrupted content in /home/jenkins/workspace/Robusta_robusta_develop-6EPNQBJK5BYEXOJV6L45MMZZGUIP7WO4Y6EGRUYNFFMRC7B2GL3A@tmp/durable-96fa79b7/pid: java.lang.NumberFormatException: For input string: ""
00:20:12.769 Cannot contact kubernetes-ef39fe82c8a541be84bd780e4d7c1ddb-ce4d47fc96bfc: java.io.IOException: corrupted content in /home/jenkins/workspace/Robusta_robusta_develop-6EPNQBJK5BYEXOJV6L45MMZZGUIP7WO4Y6EGRUYNFFMRC7B2GL3A@tmp/durable-96fa79b7/pid: java.lang.NumberFormatException: For input string: ""

Add information from comments:

The same kind of problem has occurred in my Jenkins running on GKE. This problem occurred by upgrading Pipeline Nodes and Processes Plugin from 2.8 to 2.9. I am confirming that this problem temporarily resolves by downgrading that plugin from 2.9 to 2.8.

BTW workflow-durable-task-step 2.9 does add this log message but it is just exposing a problem that was already there, and simply being suppressed unless you were running a sufficiently fine logger. The problem is that this code is seeing a file which is supposed to contain a number once created, whereas it is being created as empty for some reason.

Again the bug probably exists in all versions, it is only printed to the build log as of 2.9. You can add a FINE logger to org.jenkinsci.plugins.workflow.steps.durable_task.DurableTaskStep to verify.