Poor support from Keap. It’s not working (even if installed on just one site).

As per Keap Support, there’s nothing they can do. There is no WordPress support for it too.

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Plugin could not be activated because it triggered a fatal error.

Fatal error: Uncaught TypeError: sizeof(): Argument #1 ($value) must be of type Countable|array, bool given in /www/wp-content/plugins/infusionsoft-official-opt-in-forms/dashboard/includes/options_config.php:47 Stack trace: #0 /www/wp-content/plugins/infusionsoft-official-opt-in-forms/dashboard/includes/options.php(47): require() #1 /www/wp-content/plugins/infusionsoft-official-opt-in-forms/dashboard/dashboard.php(77): require('/www/wp-content...') #2 /www/wp-content/plugins/infusionsoft-official-opt-in-forms/dashboard/dashboard.php(46): INF_Dashboard->include_options() #3 /www/wp-content/plugins/infusionsoft-official-opt-in-forms/infusionsoft.php(83): INF_Dashboard->__construct(Array) #4 /www/wp-content/plugins/infusionsoft-official-opt-in-forms/infusionsoft.php(4213): INF_Infusionsoft->__construct() #5 /wordpress/wp-admin/includes/plugin.php(2313): include_once('/www/wp-content...') #6 /wordpress/wp-admin/plugins.php(192): plugin_sandbox_scrape('infusionsoft-of...') #7 {main} thrown in /www/wp-content/plugins/infusionsoft-official-opt-in-forms/dashboard/includes/options_config.php on line 47