I assumed that CloudBees PR builder shows the failure and that CloudBees PR builder on PR 415 shows a narrower change with the same failure, a test timeout in the KubernetesPipelineTest. Unfortunately, that assumption seems to be wrong, since the same KubernetesPipelineTest failures are visible on PR-415 even when all changes have been reverted from PR-415.
Can you confirm that and provide more details of what you learned while diagnosing that the rollback of git client plugin was the solution to the problem?
Pull request 307 seems to have previously included git client plugin 3.0.0-beta5. The most recent version is 3.0.0-beta6.
Pull request 307 includes git plugin plugin 3.9.1. The most recent version is 4.0.0-beta4. While git plugin 3.9.1 should be able to work with git client plugin 3.0.0-beta6, that is not a combination that I actively test.
I've confirmed that the kubernetes plugin unit tests run successfully with several different combinations of git client 3.0 and git plugin 4.0 beta versions. I assume the issue is not visible from the kubernetes plugin unit tests, but only in an integration test environment.