WordPress site down after installing a plugin. Can’t connect with FTP with correct details

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For sure you must solve the FTP connection since it’s not related with the plugin error.
A possible solution if you have access to phpmyadmin:
open a sql window and type:

SELECT * FROM wp_options WHERE option_name LIKE 'active_plugins'

then delete the record. Note that wp_options may change if the prefix of your tables are different, you see them all beginnnig with a prefix + underscore (wp_, or wp123_ ,or XX_,…)

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this will reset all active plugins and give you access to the site again