java.lang.NoClassDefFoundError: Could not initialize class hudson.slaves.SlaveComputer
suggests some basic problem with the agent connection. Unfortunately due to JDK-8051847 the original problem is not available in this log. It may be displayed in the agent’s own log, which can be seen in the UI, or by installing the support-core plugin and grabbing a Support bundle. Certainly an error like that would be consistent with accidentally running the agent on an unsupported version of Java—i.e., anything but 8, as in towel’s case. Whether this has anything to do with recent plugin updates, it is hard to say. I do not see any obvious connection, and towel says that indeed there was none.
The other commenters are probably encountering a totally unrelated issue, for which we have almost no diagnostics. If the error is reproducible but running with the JVM option -Dorg.jenkinsci.plugins.workflow.steps.durable_task.DurableTaskStep.USE_WATCHING=false reliably fixes it, then that would be a clear signal that it is related. The asynchronous notification was lost error is definitely related. Is this only happening for users of bat, as opposed to sh?