WordPress developers release a new version after few months and it is highly recommended to update WordPress core, plugins, and themes for better security. I tried automatic update feature but Automatic update failed for some reasons.
Front-end of my blog was working fine and I was able to view all blog posts and pages without any issue but I was not able to access WordPress Dashboard.
If automatic update does not work, You can update WordPress manually via FTP. You will have to login to your hosting control panel (cPanel etc) to update WordPress files manually.
I also updated all files manually but still I was unable to access WordPress dashboard. I am not sure if Caching plugin was causing the issue but after renaming all plugin folders, I was able to login to my WordPress dashboard.
How to rename plugin folder manually
Login to your hosting control panel. My host has cPanel. Under Files section, click on File Manager to view all your files. Now go to wp-content/plugins folder and rename all plugin folders.
You can simply add a number or a word, for example, rename jetpack to jetpackRen and akismet to akismetRen. Rename all plugins and try to login t your WordPress Dashboard, I was able to login to my Dashboard after renaming all plugin folders.
When logging in, Now rename all plugins again and remove additional number from plugin name. That’s all you need to do to fix this error.