Hi there,
Thanks for the info, I’ve also tried some testing on your site, but I still can not figure out what the cause is.
Can you enable WP debugging by adding this to wp-config.php?
define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', false );
Then retry activating the license.
Then check/wp-content/debug.log to share the log with us so we can have more info? Thanks!