I am having the same issue on HighSierra. I created log for this:
log stream --style syslog --predicate 'senderImagePath contains[cd] "TimeMachine"' --info
Filtering the log data using "senderImagePath CONTAINS[cd] "TimeMachine""
Timestamp (process)[PID]
2018-10-06 00:05:00.697923+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Starting manual backup
2018-10-06 00:05:01.263394+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Backing up to /dev/disk2s2: /Volumes/Adata HD710/Backups.backupdb
2018-10-06 00:05:01.902425+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to remove attribute 'com.apple.backupd.SnapshotVolumeFSEventStoreUUID' from 'file:///', error: Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted"
2018-10-06 00:05:01.902719+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to remove attribute 'com.apple.backupd.SnapshotVolumeLastFSEventID' from 'file:///', error: Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted"
2018-10-06 00:05:01.902833+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to remove attribute 'com.apple.backupd.SnapshotVolumeUUID' from 'file:///', error: Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted"
2018-10-06 00:05:01.902943+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to remove attribute 'com.apple.backupd.PreviousSnapshotVolumeUUID' from 'file:///', error: Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted"
2018-10-06 00:05:01.904551+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to remove attribute 'com.apple.backupd.PreviousSnapshotVolumeName' from 'file:///', error: Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted"
2018-10-06 00:05:01.904861+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to remove attribute 'com.apple.backupd.VolumeBytesUsed' from 'file:///', error: Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted"
2018-10-06 00:05:01.905049+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to remove attribute 'com.apple.backupd.VolumeIsCaseSensitive' from 'file:///', error: Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted"
2018-10-06 00:05:01.905460+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to remove attribute 'com.apple.backupd.fstypename' from 'file:///', error: Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted"
2018-10-06 00:05:01.905631+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to remove attribute 'com.apple.backupd.VolumeIsEncrypted' from 'file:///', error: Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted"
2018-10-06 00:05:01.914808+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Starting age based thinning of Time Machine local snapshots on disk '/'
2018-10-06 00:05:01.924310+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Completed thinning of snapshots on disk '/' in 0.0 seconds - current free space: 116,55 GB (116 551 569 408 B), target free space: 25 GB (25 000 000 000 B), initial free space: 116,55 GB (116 551 569 408 B), urgency: 0, remaining snapshots: (
"com.apple.TimeMachine.2018-10-05-105037"
)
2018-10-06 00:05:03.182964+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to write unlock record UUIDs as extended attribute for disk '/', error: Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted"
2018-10-06 00:05:03.280591+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Created Time Machine local snapshot with name 'com.apple.TimeMachine.2018-10-06-000503' on disk '/'
2018-10-06 00:05:03.288148+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Declared stable snapshot: com.apple.TimeMachine.2018-10-06-000503
2018-10-06 00:05:03.518980+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Mounted stable snapshot: com.apple.TimeMachine.2018-10-06-000503 at path: /Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/MacBook Pro (Grzegorz)/2018-10-06-000503/Macintosh source: Macintosh
2018-10-06 00:05:03.811376+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Initial backup of source: "Macintosh" (device: /dev/disk1s1 mount: '/' fsUUID: A667477E-764D-4E23-BB80-88BCEF9C7C87 eventDBUUID: 0282DECA-678A-43E2-8B7C-14973EC1D394)
2018-10-06 00:05:04.119454+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Found 771177 files (125,95 GB) needing backup
2018-10-06 00:05:04.174070+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] 147,84 GB required (including padding), 597,34 GB available
2018-10-06 00:07:29.354539+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Error: (-48) SrcErr:NO Copying /Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/MacBook Pro (Grzegorz)/2018-10-06-000503/Macintosh/private/var/folders/zz/zyxvpxvq6csfxvn_n 00000y800007k/0/com.apple.nsurlsessiond/1E63E8DC1D830BBB0C56F20CBC3704604A8057D0 /14F8026CDF3C4E7FA1E21ACC1B0569499AB84464 to /Volumes/Adata HD710/Backups.backupdb/MacBook Pro (Grzegorz)/2018-10-06-000503.inProgress/C969ED15-B1F6-4AC2-968E-3F6F4378C4FE/Ma cintosh/private/var/folders/zz/zyxvpxvq6csfxvn_n00000y800007k/0/com.apple.nsurls essiond/1E63E8DC1D830BBB0C56F20CBC3704604A8057D0
2018-10-06 00:07:29.354945+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Stopping backup.
2018-10-06 00:07:29.356134+0200 localhost backupd[17059]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Error: (-8062) SrcErr:NO Copying /Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/MacBook Pro (Grzegorz)/2018-10-06-000503/Macintosh/private/var/folders/zz/zyxvpxvq6csfxvn_n 00000y800007k/0/com.apple.nsurlsessiond/1E63E8DC1D830BBB0C56F20CBC3704604A8057D0 /14F8026CDF3C4E7FA1E21ACC1B0569499AB84464/configuration.plist to /Volumes/Adata HD710/Backups.backupdb/MacBook Pro (Grzegorz)/2018-10-06-000503.inProgress/C969ED15-B1F6-4AC2-968E-3F6F4378C4FE/Ma cintosh/private/var/folders/zz/zyxvpxvq6csfxvn_n00000y800007k/0/com.apple.nsurls essiond/1E63E8DC1D830BBB0C56F20CBC3704604A8057D0