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have you tried:
-deactivating ALL plugins temporarily to see if this resolves the problem. If this works, re-activate them individually (one-by-one) to find the problematic plugin(s). If you can’t get into your admin dashboard, try deactivating via FTP or SFTP or whatever file management application your host provides. If applicible, also remember to deactivate any plugins in the mu-plugins folder. The easiest way is to rename that folder to mu-plugins-old.
– To rule out any theme-specific issue, try switching to the unedited default theme for a moment using the WP dashboard. If you don’t have access to your admin area, you can switch to the default theme by renaming your current theme’s folder in wp-content/themes and adding “-old” to the end of the folder name using via FTP or SFTP or whatever file management application your host provides. Alternately, you can remove other themes except the default theme. That will force your site to use it.

no.i havent..Im trying to install it on my localserver as the website is live and its a client site..So,need to make it from a localserver to see the issues.

Hello, I did that..And didnt work..All plugins are not working.So,I guess that means they were deactivated..After, I dont see any mu-plugins..And I change a theme to a default one twenty-eleven still didnt work…

Any idea how to fix it??

Do you see the admin tool bar? If you see the admin tool bar, you are (to some degree) logged in. try logging out and deleting all your site cookies in your browser.

If tha does not help, try resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems (because the hooks remain unless plugins completely removed or some plugins stick around in cached files. So by renaming the folder, you break them and force them inactive).

Hello,

thanks for the reply..I try that one on using the file manager or php myadmin..But no luck..Still didnt work…Even changing the plugin to plugins-2, didnt work, I also try this method

“In the table wp_options, under the option_name column (field) find the active_plugins row
Change the option_value field to: a:0:{}”

But it didnt work…When I was doing it..I also try to input on the browser https://www.younescosmeticdentist.com/wp-admin/network/ but it still gives me the error..

Any idea on how to fix this?