Reading in reverse order, the temporary file deletion and comment about synthetic filelist indicate that the previous backup was interrupted. The fact that it mentions a dindex file rather than expected dlist may be caused by a suspected bug. The change journal comment makes me wonder if --usn-policy option is set even though it only works on local NTFS drives (not mapped network drives). You could try removing that.
The big question is unresolved. If it’s just sitting there showing no more, maybe raise log level to Profiling?
SMB sometimes does weird things, especially if Duplicati is a Windows service (which is not the default). Maybe I should have asked for Verbose so you could at least see one line per file as it scans source area, however I’m not sure if it would have announced that 1409 file count if it hadn’t actually looked over files…