unsuccessful installation of DSM 6.1.6-15266 on my Synololgy DiskStation DS713+

• https://www.synology.com/en-uk/releaseNote/DS713+

I do not succeed installing DSM 6.1.6-15266 on my DS713+:

Unknown error occurs (21).

This has happened repeatedly since Thursday/Friday midnight.

Update: rebooted, re-downloaded the update, update succeeded.