Fatal error when login in

Hello,
I’m using Nextend and Buddypress on my site, but users register and log in through the theme’s ajax form. This setup has worked well until about 15 days ago, when I started getting the error.
Now, every time a user registers, either with Google or Facebook, I get a WP email reporting the fatal error bellow and the user gets this https://prnt.sc/s92ms1

Thanks in advance

Error details
================
An error of type E_ERROR was caused in line 91 of the file /var/www/vhosts/xxxx/httpdocs/wp-content/plugins/buddypress/bp-members/bp-members-activity.php. Error message: Uncaught Error: Call to undefined function bp_activity_add () in /var/www/vhosts/xxx/httpdocs/wp-content/plugins/buddypress/bp-members/bp-members-activity.php:91
Stack trace:
# 0 /var/www/vhosts/xxxx/httpdocs/wp-content/plugins/nextend-facebook-connect/includes/user.php(407): bp_core_new_user_activity (91)
# 1 /var/www/vhosts/xxxxx/httpdocs/wp-includes/class-wp-hook.php(287): NextendSocialUser-> registerComplete (91)
# 2 /var/www/vhosts/xxxxx/httpdocs/wp-includes/class-wp-hook.php(311): WP_Hook-> apply_filters (NULL, Array)
# 3 /var/www/vhosts/xxxxxx/httpdocs/wp-includes/plugin.php(478): WP_Hook-> do_action (Array)
# 4 /var/www/vhosts/xxxxxx/httpdocs/wp-includes/user.php(1907): do_action (‘user_register’, 91)
# 5 /var/www/vhosts/xxxxxx/httpdocs/wp-content/plugins/nextend-facebook-connect/includes/user.php(334): wp_insert_user (Array)
# 6 /var/www/vhosts/xxxxxx/httpdocs/wp-content/plugins/nextend-facebook-connect/includes/user.php(112): NextendSocialUser-> register (