Failure upgrading / updating site to WordPress 4.7

I’ve just clicked to upgrade my site from WP 4.6.x to 4.7, but it has failed with little clue of what’s wrong.

The front end still loads and works fine, but when I try and login to the back end, it sends me to this empty page:

http://www.example.com/wp-admin/upgrade.php?_wp_http_referer=%2Fwp-admin%2F

I’ve checked here, but when I log in with FTP there is no .maintenance file to delete.

Also, looking in the wp-admin folder, there does not appear to be an upgrade.php file that the above URL seems to refer to.

Any ideas on how I can get back access to the backend again and/or how to complete the upgrade successfully?

PS – I last updated my plugins only a few hours ago and the site was working fine then.

Edit

I’ve turned on WP_DEBUG in my wp-config.php file, and when I try to login I am now taken to…

http://www.example.com/wp-admin/

…which gives this output…

Notice: The called constructor method for WP_Widget in Woo_AdWidget is
deprecated since version 4.3.0! Use

__construct()

instead. in /home/public_html/example.com/wp-includes/functions.php on
line 3846

Notice: The called constructor method for WP_Widget in
Woo_BlogAuthorInfo is deprecated since version 4.3.0! Use

__construct()

instead. in /home/public_html/example.com/wp-includes/functions.php on
line 3846

Notice: The called constructor method for WP_Widget in Woo_EmbedWidget
is deprecated since version 4.3.0! Use

__construct()

instead. in /home/public_html/example.com/wp-includes/functions.php on
line 3846

Notice: The called constructor method for WP_Widget in Woo_flickr is
deprecated since version 4.3.0! Use

__construct()

instead. in /home/public_html/example.com/wp-includes/functions.php on
line 3846

Notice: The called constructor method for WP_Widget in Woo_Subscribe
is deprecated since version 4.3.0! Use

__construct()

instead. in /home/public_html/example.com/wp-includes/functions.php on
line 3846

Notice: The called constructor method for WP_Widget in
Widget_Woo_Portfolio is deprecated since version 4.3.0! Use

__construct()

instead. in /home/public_html/example.com/wp-includes/functions.php on
line 3846

Notice: The called constructor method for WP_Widget in
Woo_Widget_Component is deprecated since version 4.3.0! Use

__construct()

instead. in /home/public_html/example.com/wp-includes/functions.php on
line 3846

Notice: The called constructor method for WP_Widget in
dk_speakup_petition_widget is deprecated since version 4.3.0! Use

__construct()

instead. in /home/public_html/example.com/wp-includes/functions.php on
line 3846

Notice: The called constructor method for WP_Widget in
search_FAQ_Widget is deprecated since version 4.3.0! Use

__construct()

instead. in /home/public_html/example.com/wp-includes/functions.php on
line 3846

Notice: The called constructor method for WP_Widget in
random_FAQ_Widget is deprecated since version 4.3.0! Use

__construct()

instead. in /home/public_html/example.com/wp-includes/functions.php on
line 3846

Notice: The called constructor method for WP_Widget in
recent_FAQ_Widget is deprecated since version 4.3.0! Use

__construct()

instead. in /home/public_html/example.com/wp-includes/functions.php on
line 3846

Notice: The called constructor method for WP_Widget in
topics_FAQ_Widget is deprecated since version 4.3.0! Use

__construct()

instead. in /home/public_html/example.com/wp-includes/functions.php on
line 3846

Notice: The called constructor method for WP_Widget in
cloud_FAQ_Widget is deprecated since version 4.3.0! Use

__construct()

instead. in /home/public_html/example.com/wp-includes/functions.php on
line 3846

Warning: Cannot modify header information – headers already sent by
(output started at
/home/public_html/example.com/wp-includes/functions.php:3846) in
/home/public_html/example.com/wp-includes/pluggable.php on line 1179

Try disabling your plugins directory. I’ve been fixing this repeatedly today. Disabling the plugin directory has, for me, allowed the page to load and complete the upgrade process. I suspect it is a specific plugin doing it but haven’t determined which yet.

Update

If you have W3 Total Cache 9.5.1 installed, deactivate it and try again. That has been the common factor and what is throwing errors in our error logs. Disable it, run the upgrade, enable it again.