Getting error=unable to get issuer certificate with SSL VPN and Comodo certificate on OS X

Hi,

I'm using the XG firewall as SSL VPN server. I bought a Comodo Positive SSL certificate for a .org domain (example utm.xxxx.org). I'm using this certificate for the SSL VPN server configured in this way:

Protocol: UDP

SSL server certificate: utm.xxxx.org

Override Hostname: utm.xxxx.org

Enc Alg: AES 128 CBC

Auth Alg: SHA2 256

Key size: 2048

Key Lifetime: 28800

Compression: On

Using user andrea, the connection works on Windows with Sopshos SSL VPN client, on Android with OpenVPN app, on iPad with OpenVPN app but do not work on OS X 10.12.6 using tunnelblinck (stable 3.7.5a and beta 3.7.6beta04). I used this guide for the VPN configuration.

The error I'm getting is:

2018-05-04 12:31:54 VERIFY ERROR: depth=2, error=unable to get issuer certificate: C=GB, ST=Greater Manchester, L=Salford, O=COMODO CA Limited, CN=COMODO RSA Certification Authority
2018-05-04 12:31:54 OpenSSL: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed
2018-05-04 12:31:54 TLS_ERROR: BIO read tls_read_plaintext error
2018-05-04 12:31:54 TLS Error: TLS object -> incoming plaintext read error
2018-05-04 12:31:54 TLS Error: TLS handshake failed

Using  SFOS 17.0.6 MR-6 virtual appliance.

Best regards

Andrea