Hey, so I found the problem:
This line:
printf( esc_html__( 'Hey, we noticed you just crossed %1$s submissions%2$s on this %3$s - that\'s awesome! We have spent countless hours developing this free plugin for you, and we would really appreciate it if you could drop us a rating on wp.org to help us spread the word and boost our motivation.', 'forminator' ), '<strong> ' . (int) $milestone, '</strong>', esc_html( static::$module_slug ) );
Specifically the \ sign.
