Only the server software on the nucleus. As has been reported by several other roon users on your forum.
SEE:
https://community.roonlabs.com/t/roon-server-software-is-restarting-frequently/115460
https://community.roonlabs.com/t/roon-v1-6-roon-software-server-constantly-reboots/60820
https://community.roonlabs.com/t/roon-server-software-keeps-resetting/177727
So I am not the only user experiencing this problem.
FOLLOW UP
I have removed the path to the (both the WD and the Synology) NAS (in Roon app/Settings/Storage) and now (at least since a couple of hours) teh problem does NOT occur anymore.
So the continuous reboot happens when the nucleus tries to acces the (both) NAS.