These errors may appear if agent is switched to use new selective sync on Windows. These are generic errors returned by the system and basically mean that some unexpected error occurred when creating or connecting to a selective sync folder. Phrase "sync root" is a system's term that means the root directory for creating placeholders. While error messages can be different, below are Resilio Agent's error codes and the least possible reasons that caused that.
Cloud Filter API is not available
system requirements are not met. New Selective Sync only works on Windows 10 build 1809 and newer Windows Server 2019 build 1809+. Also requires Cldapi.dll, located in C:\Windows\System32.
Failed to initialize sync root manager registry
Failed to register sync root
Failed to unregister sync root
Agent was not able to register or unregister key \HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\SyncRootManager\
Perhaps, the Agent runs under a very restricted user account that does not have permission to edit Windows registry.
Another possible reason is that the agent was set to use an invalid path for SS job first, and then switched to new one that is supposed to work. Such a switch won't work and agent needs to be first removed from the job, and then added back with the correct path.
Failed to connect to sync root
Failed to disconnect from sync root
Possible reasons and fixes:
1) this is not an NTFS file system. New Selective Sync only works on NTFS file system.
2) this is some non-local drive. It's not possible to use new Selective Sync for a network share. Though. the selective sync folder is available remotely through SMB.
3) some other app or Explorer.exe keeps exclusive handle to the directory. For example, pointing Selective Sync agent and OneDrive to same directory won't work. To find such an app, you might want to use Process Monitor tool and check which process works in this directory and with which access.
4) two Resilio Agents run on same computer that are set to selectively sync same directory or a nested folder.