SSL Certificate Problem

Hi,

Fortinet firewall installed to our network 1 week ago. Firewall gives "Your connection is not private" for our company website although it has valid SSL cerficate from Let's Encrypt (valid for 3 months). We faced with this problem after firewall installed.

So, what is the problem? Fortinet firewall doesn't trust to free of charge SSL certificate providers like Let's Encrypt? Our company web site has "A rating" at www.ssllabs.com .

Best Regards.