TimeMachine backup not possible

Hi group,

since updating to High Sierra, I am no longer able to run time machine backups.

Setup as follows:

* MacBookPro 10.13.3

* System Language German

* Network attached Storage "WD MyBookLive" attached to LAN

* MacBook is connected to mains all the time

* local drive is 250 GB, 23.5 GB "free", 12.5 GB "available" (according to finder)

* Avast Security is installed

I created a new share on the MyBookLive and advertised it to be suitable for time machine backups.

Time Machine backup finds that device and authentication works out.

I tried both a public and a private share, no difference.

When the backup is run, I see the following messages in the log file:

2018-03-24 19:13:00.278669+0100 localhost backupd[78967]: (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"

2018-03-24 19:13:12.989539+0100 localhost backupd[78967]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Copy stage failed with error:11

2018-03-24 19:13:13.033800+0100 localhost backupd[78967]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to unmount disk mounted at '/Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/mbp-michael/201 8-03-24-191300/Macintosh HD', error: {

Action = Unmount;

Target = "file:///Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/mbp-mich ael/2018-03-24-191300/Macintosh%20HD";

}

2018-03-24 19:13:13.035091+0100 localhost backupd[78967]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to unmount snapshot: com.apple.TimeMachine.2018-03-24-191300 source: Macintosh HD

2018-03-24 19:13:13.037674+0100 localhost backupd[78967]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Backup failed because a backup source disk was ejected or disconnected unexpectedly!

2018-03-24 19:13:13.040191+0100 localhost backupd[78967]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Backup failed with error 37: 37

As I read about issues with not being able to access files due to other processes, I already tried to disable spotlight indexing and Avast Security.

Note that neither was required in Sierra.

Note that neither of these solved the issue.

I'm basically stuck with a system without a backup, it's only got its internal HDD.

As I'll be travelling on monday evening (Europe time), I'd like to get a backup up and running before that.

Do you have any suggestions on how to solve this problem?

Best regards

Michael