Summary: The syscall filter daemon option flag -F is the cure. Using '-F 0' to disable the syscall filter works. No other settings are workable.
A summary table: Chrony -- daemon flags used -- Version -F0 -F1 -F-1 ------- ------ ------ ------ 4.0-9 ok SIGSYS SIGSYS #83f96e ok SIGSYS SIGSYS Using the latest '/etc/systemd/system/chronyd.service' from 'examples' subdirectory in the Git repo does not improve the picture nor alter the summary table given above. chrony: 4.0-8, Debian chrony: git repo "development" main HEAD branch (#83f96efd), Sept 29, 2021 libseccomp2 2.5.1-1 libc6 2.31-13 Kernel: 5.10.46 - untainted Virtualization: non-virtualized, machine-level CPU: Intel i5-3470
