Auto Backup failed to copy file

[JackBassV]

For some reason, auto backup has been failing occasionally. Manual  backup works fine.
Got this warning:

java.lang.Exception: Failed to copy file 'C:\Users\username\AppData\Roaming\Azureus\plugins\azneti2phelper\netDb\rK\routerInfo-Kh0juK~fo5Ajy1AhONl4U34S9-Sxr1joFWEApXJkqHc=.dat'
    at com.aelitis.azureus.core.backup.impl.BackupManagerImpl.copyFilesSupport(BackupManagerImpl.java:638)
    at com.aelitis.azureus.core.backup.impl.BackupManagerImpl.copyFilesSupport(BackupManagerImpl.java:618)
    at com.aelitis.azureus.core.backup.impl.BackupManagerImpl.copyFilesSupport(BackupManagerImpl.java:618)
    at com.aelitis.azureus.core.backup.impl.BackupManagerImpl.copyFilesSupport(BackupManagerImpl.java:618)
    at com.aelitis.azureus.core.backup.impl.BackupManagerImpl.copyFilesSupport(BackupManagerImpl.java:618)
    at com.aelitis.azureus.core.backup.impl.BackupManagerImpl.copyFiles(BackupManagerImpl.java:584)
    at com.aelitis.azureus.core.backup.impl.BackupManagerImpl.backupSupport(BackupManagerImpl.java:845)
    at com.aelitis.azureus.core.backup.impl.BackupManagerImpl.access$600(BackupManagerImpl.java:68)
    at com.aelitis.azureus.core.backup.impl.BackupManagerImpl$7.runSupport(BackupManagerImpl.java:553)
    at org.gudy.azureus2.core3.util.AsyncDispatcher$1.run(AsyncDispatcher.java:160)
    at org.gudy.azureus2.core3.util.AEThread2$threadWrapper.run(AEThread2.java:294)

No idea why. All files exist, as does the backup folder. And most of the time it seems to work: ie, I get no warnings.

Vuze64 5711b02, Java 9b102x64, Win10