Update process went wrong - how to proceed?

Nextcloud version: 15.0.8
Operating system and version: synology
Apache or nginx version: apache 2.4
PHP version: 7.0.33

The issue you are facing:
The update did not fully finish. The last part of the updater.log is:

2019-07-15T00:32:04+0200 0XrhNxQM5s [info] startStep("9")
2019-07-15T00:32:04+0200 0XrhNxQM5s [info] deleteOldFiles()
2019-07-15T00:32:40+0200 0XrhNxQM5s [info] config sample exists
2019-07-15T00:32:40+0200 0XrhNxQM5s [info] themes README exists
2019-07-15T00:33:34+0200 0XrhNxQM5s [info] end of deleteOldFiles()
2019-07-15T00:33:34+0200 0XrhNxQM5s [info] endStep("9")
2019-07-15T07:55:12+0200 Uf8XBrqWCU [info] request to updater
2019-07-15T07:55:12+0200 Uf8XBrqWCU [info] currentStep()
2019-07-15T07:55:12+0200 Uf8XBrqWCU [info] Step 9 is in state "end".
2019-07-15T07:55:12+0200 Uf8XBrqWCU [info] show HTML page
2019-07-15T07:55:12+0200 Uf8XBrqWCU [info] current version: 15.0.8 build time:

When I do:
sudo -u http /usr/local/bin/php70 /volume1/owncloud/updater-oc00qcm0h0mx/downloads/nextcloud/occ

I get:

The current PHP memory limit is below the recommended value of 512MB.
Nextcloud is not installed - only a limited number of commands are available
Nextcloud 15.0.10

It seems that I still have a backup of my previous config:
# ls /volume1/owncloud/updater-xxxxxxxxxxxxx/backups/nextcloud-
3rdparty apps AUTHORS config console.php COPYING core cron.php index.html index.php lib occ ocm-provider ocs ocs-provider public.php remote.php resources robots.txt settings status.php themes updater version.php

These files are not in /volume1/web/nextcloud/

There are only these files:

# ls /volume1/web/nextcloud/
apps  config  index.php  public.php  remote.php  status.php  themes  updater

How should I proceed to fix this? Should I copy the backup back to /volume1/web/nextcloud/?

When I try to access nextcloud via the web interface, I get a page with this text:
Update in process.

I took the risk and now answering myself:
I copied aside the /volume1/web/nextcloud directory, and copied here the backed up files from /volume1/owncloud/updater-oc00qcm0h0mx/backups/nextcloud-

I made sure all file permissions and ownership stayed the same.

The result is that my old version (15.0.8) is accessible again, and up and running fine, as far as I could see for now.

I guess that the problem with the web update was probably caused by server timeouts. See: https://github.com/nextcloud/updater/issues/212#