Server Errors after activating the plugin

Hello, after activating the plugin on WP-admin backend, first the whole page went blank. After that a html500 error was shown, and then the website went extremely slow.

I cannot even log-in into my wp-admin backend, as the “redirect_to=http%3A%2F%2F” message was shown and kept me re-looping the log-in page. I have tried to delete the htaccess, deactivated ALL plugins in FTP, and also removing the “WP Encrypt” in FTP, but still fails to restore the site.

In addition, I found that all of my other sites under the same server got the problem for “redirect_to=http%3A%2F%2F” message.

Would like to seek help, many thanks!!