Update Fehler: could not unlink .../Updater/.../index.html

Nextcloud version : 15.05
Operating system and version (eg, Ubuntu 17.04): Ubuntu

The issue you are facing:
Updating from 15.05 to 15.06 with the web-updater stops at “Delete old files” with the massage:
Could not unlink: /var/www/updater/…/index.html
Retry update doesn’t help.

In the folder updater there are only two files: index.php and updater.phar

Is this the first time you’ve seen this error? (Y/N): Y

Steps to replicate it:

Couldn’t respart the update

Who can help?