ERROR Error starting file metadata download thread

This one has been causing me problems for awhile.

Server: 2.4.15 docker image on Ubuntu Server 20.04 Host
Client: Various are affected, but most common failure is on Mac Monterey client 2.4.12

A persistent, but not constant problem is backups fail to run. This mainly happen with my Mac Monterey client. The logs typically look like this

26/06/22 17:38
Starting unscheduled incremental file backup...
Info
26/06/22 17:42
Backing up "rootfs" without snapshot.
Info
26/06/22 17:42
Indexing of "rootfs" done. 209056 filesystem lookups 0 db lookups and 0 db updates
Info
26/06/22 17:42
domsmacbook: Loading file list...
Info
26/06/22 17:42
domsmacbook: Calculating file tree differences...
Info
26/06/22 17:42
domsmacbook: Creating snapshot...
Info
26/06/22 17:42
domsmacbook: Deleting files in snapshot... (4286)
Info
26/06/22 17:43
domsmacbook: Deleting files in hash snapshot...(4286)
Info
26/06/22 17:43
Waiting for metadata download stream to finish
Errors
26/06/22 17:44
Error starting file metadata download thread
Errors
26/06/22 17:44
Backup had an early error. Deleting partial backup.

type or paste code here

e.g. the process gets to the point of deleting files in the hash snapshot (and possibly gets passed that), and is followed by Waiting for metadata download stream to finish and Error starting file metadata download thread

This happens very frequently on my main Mac client. I have also seen it happen when backing up my Windows client, and the ubuntu server. The error is rarer on the Windows client and Ubuntu server though.

It would be great to get some tips to prod this and track down the issue.

Thanks in advance.