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what hypervisor are you using?

vSphere, but we do not have any of the Citrix<->vCenter integration configured.

You comb the event logs first on the VDA and DDC at the time of reboots? I might spend a few minutes with that, and then just roll back the VDA to get back to square and confirm that was root cause.

Yes, no errors/warnings on the VDAs. Rolling back the DDCs to CU2 resolves it.

you do an upgrade of the VDA or completely uninstall/reinstall?

The issue was noticed with the DDCs on 1912 CU5 and the VDAs still on 1912 CU2. I upgraded a VDA vDisk to CU5 by uninstalling and then running the VDA cleanup utility, and the issue persisted. As soon as I rolled back the DDCs to CU2 all VDAs (even the ones on CU5) reboot properly. For some reason, after the first half of a DG restarts the DDCs never see that (even though the VDAs register) so it never moves on to the second half of the DG and eventually times out. I plan to try 1912 CU4 on the DDCs to see how it behaves. Can't try 2203 yet due to SQL limitations.

well what the heck, shouldn't be the jump from CU2 to CU5, but i wonder if it is