I had my Ubuntu 15.10 system working just fine, and Wordpress updates happened over ssh. But when I upgraded Ubuntu to 16.04, the switch to php7.0 broke Wordpress updates. At first it was prompting for FTP login credentials until I installed php-ssh2 (which cautions that it's based on a git snapshot). Now when I try to upgrade a plugin, it says

An error occurred while updating WP to Twitter: Unable to locate WordPress Content directory (wp-content).

And in /var/log/apache2/error.log:

[Sun Jun 12 11:01:10.969931 2016] [:error] [pid 5545] [client 192.168.1.11:65278] PHP Warning:  file_put_contents(): Unable to open ssh2.sftp://Resource id #81/.maintenance on remote host in /home/blog/wordpress/wp-admin/includes/class-wp-filesystem-ssh2.php on line 252, referer: http://xxxx.com/wp-admin/update-core.php?action=do-plugin-upgrade

[Sun Jun 12 11:01:10.970772 2016] [:error] [pid 5545] [client 192.168.1.11:65278] PHP Warning:  file_put_contents(ssh2.sftp://Resource id #81/.maintenance): failed to open stream: operation failed in /home/blog/wordpress/wp-admin/includes/class-wp-filesystem-ssh2.php on line 252, referer: http://xxxx.com/wp-admin/update-core.php?action=do-plugin-upgrade

Am I missing a package, or a configuration item, or what?