[video explanation] can't login and can't upgrade manually

Edit
For those who don’t want/cant see the video:
The issue is not being able to login.
In the video I show 4 steps I took to resolve my issue, all of them fail:
1- upgrade manually by copying over all new WP 4.5.1 files to my existing WP 4.4.2 (excluding wp-config.php and wp-content folder).
Result: blog fails to load with an error in the wp-widgets.php file.

2- upgrade manually by copying only wp-admin folder. Blog loads, but same login failure issue.

3- upgrade manually by copying only wp-admin and wp-includes folder (excluding all root folder files and wp-content folder).
Result: blog fails to load with an error in the default-widgets.php file.

4- upgrade manually by copying only new wp-admin folder and new wp-settings.php file .
Result: blog fails to load with an error in the default-widgets.php file.

Thanks

I hope someone can help please !
I am ready to pay for the service if required

My guess is that there is a plugin or theme that is causing problems. Upgrading WP wont fix it. You should disable all your plugins and then try changing to a default theme. To disable plugins, you need sftp or ssh access to your server. Just rename the plugins directory:

/wp/wp-content/plugins.bak
/wp/wp-content/mu-plugins.bak

See if you can login after renaming the plugins directories.

Thanks jkhongusc. I could login after I renamed the plugins folder (strangely, renaming the plugins folder to “_plugins” didn’t result in excluding it, and apparently it was still causing the problem , only after I renamed it to plugins.bak that WordPress started not seeing it and I could login.
However, I still can’t upgrade WordPress, neither from the dashboard automatically or by uploading the new version (the upload button simply doesn’t respond), nor by upgrading via accessing the site’s files via FTP and copying over the new files replacing the old ones. That still results in the errors I explained above and in the video.

Did you try selecting a default theme – like twenty fifteen? Then try upgrading WP. If all else fails, you can manually upgrade WP – download the package then copy the files over manually.

You can’t select a theme for the WP Network’s dashboard.
I can select a theme for the main site.
But going to the WP Network dashboard, all themes appear “disabled” (Although the sites load fine with the default theme).
No matter how many times I click “enable” , the themes just remain disabled in the dashboard, and I can’t switch to any other theme.
If I completely delete the folder of the default (active) theme, the other themes would remain “disabled” and the sites would not load at all !!

okay, after deleting it, the dashboard of each site reverted to twenty-fourteen theme (strangely, not twenty fifteen). I am noting that all 3 default WP themes still appear “disabled” in the network dashboard, although when I click “enable” any one of them, I see “theme enabled” note.
In any case, I changed the theme of the main site to twenty fourteen, but still the network dashboard shows that I am using WP 4.0 , and still says “you are using the latest version of wordpress” !!!

Are you sure you are going to the Network Installed Themes menu. The url looks like this – domain.com/wp-admin/network/themes.php

I am not sure how you got to this point. I would manually upgrade WP and hope that it works better. It is not that hard.

If it is a newer site, you might consider starting from scratch.

Yes, sure, it’s the Network themes page.
And I already tried to upgrade manually, but this is the major issue I am having, like I explained above.

If you upgrade WP manually, what is wrong at that point?

IMO there is an incompatibility between WP 4.5 and your plugins & themes.

You should rename your mu-plugins directory too; or check it to make sure there are no files/folders in there… /wp-content/mu-plugins.bak