After an update this issue re-appeared, in our special configuration
Jenkins 2.32.2
Libvirt Plugin with 8 virtual machines at one and 4 times 2 virtual machines at 4 more real servers
We suffer from issue 32118 (https://issues.jenkins-ci.org/browse/JENKINS-32118), therefore we added the following Java option at our startup script: \"-Dhudson.model.DirectoryBrowserSupport.CSP=sandbox allow-scripts; default-src 'none'; img-src 'self'; style-src 'self';\"
After a restart, all our nodes stay offline (the virtual machines get started), because of this error on the clients
Downgraded to 1.611, and issue is resolved.