I have the same problem. I am using Ubuntu 14.04 with PHP 7.1
Nextcloud resides in /var/customers/webs/sysu/cloud/, open_basedir is set to "/var/customers/webs/sysu/cloud:/var/customers/tmp/sysu:/usr/share/php:/usr/share/php5:/tmp:/var/customers/tmp/sysu/:/dev/urandom"
It tries to access “/var/customers/webs/sysu/cloud/updater/…/…”, which fails.
When I change the open_basedir to include “/var/customers/webs/sysu/”, the updater works.