Errors like this are logged. Check the error log on your server. If you can’t find the log, please contact your host.
Meantime, enable wp_debug and wp_debug_log and after an error, look at wp-content/debug.log to see if anything gets logged there. https://codex.ww.wp.xz.cn/Debugging_in_WordPress
You can also try this: Please attempt to disable all plugins, and use one of the default (Twenty*) themes. If the problem goes away, enable them one by one to identify the source of your troubles. http://www.wpbeginner.com/plugins/how-to-deactivate-all-plugins-when-not-able-to-access-wp-admin/
For future people with this same problem. This post helped me:
vashistha (@vashistha)
2 months, 2 weeks ago
How I solved my WordPress websites update problem in 2 Minutes
upgrade.php?_wp_http_referer=%2Fwp-admin%2F
After wordpress update I was redirected to following URL and Blank Page
Here is the step by step EASY process How I solved my problem very easily
1. Open File manager of your webhosting account
2. Rename Plugin File to Plugin123
3. Visit your website /wp-admin page
Your browser will automatically redirected to this page
Click on the update wordpress Database Page and once the database is full updated go back to File manager and rename your plugin folder.
Alternative Method to Solve upgrade.php?_wp_http_referer=%2Fwp-admin%2F method
Get the latest WordPress zip (or tar.gz) file.
Unpack the zip file that you downloaded.
Deactivate plugins.
Delete the old wp-includes and wp-admin directories on your web host (through your FTP or shell access).
Using FTP or your shell access, upload the new wp-includes and wp-admin directories to your web host, in place of the previously deleted directories.
Upload the individual files from the new wp-content folder to your existing wp-content folder, overwriting existing files. Do NOT delete your existing wp-content folder. Do NOT delete any files or folders in your existing wp-content directory (except for the one being overwritten by new files).
Upload all new loose files from the root directory of the new version to your existing wordpress root directory.
http://www.vashistha.in/2016/12/problem-solved-upgradephpwphttpreferer2.html
