This got my attention:
Jan 15 19:17:40 xcp-ng-xen12-lon2 xapi: [error||623653 INET :::80||import] Caught exception in import handler: VDI_IO_ERROR: [ Device I/O errors ]
Jan 15 19:17:40 xcp-ng-xen12-lon2 xapi: [error||623653 INET :::80||backtrace] VDI.import D:378e6880299b failed with exception Unix.Unix_error(Unix.EPIPE, "single_write", "")
Jan 15 19:17:40 xcp-ng-xen12-lon2 xapi: [error||623653 INET :::80||backtrace] Raised Unix.Unix_error(Unix.EPIPE, "single_write", "")
This Unix.EPIPE error on the remote target means that the pipe stream is being closed before VDI.Import receives all the data. The outcome is a VDI I/O error due to a broken, partial sent/received VDI.
Since a remote-over-the-internet link can be more prone to latency/intermittency issues, it might be needed to adjust the remote NFS soft timeout/retries or mounting the target with hard option.
I would also check if the remote target is running out-of-space during the backup process.