Hannes Kogler I sincerely regret that I did not detect the ISO date parsing problem in git plugin 2.3.5 which was exposed by git client plugin 1.17.0. Unfortunately, I was running an unreleased version of the git plugin on the systems where I performed my tests. I'll be more thorough prior to future releases.
I agree with you that it is a serious problem to tell users that they need to downgrade their plugin. I'm not sure any workspace cleanup is required, but at least one user has reported that it was required for him. Did downgrading the plugin work for you (without cleaning the workspace)?
The most direct path to resolve the problem is to release a new version of the git plugin, a version which already contains the fix for the problem exposed by git client plugin 1.17.0. That means the new version of the git plugin needs more testing, lest recent changes to it cause more problems for the 50,000+ installations of the plugin. I've posted a pre-release version of the git plugin 2.3.6. I'd love to hear your feedback to confirm that it resolves this issue and does not introduce other, more serious issues. I'm running that build on my test machine currently.