While PositiveSSL works wonderfully on Sophos SG, customers on Sophos XG report problems with PositiveSSL wildcard certificates, the validity of which is not accepted despite importing all the necessary root CA and intermediate certificates.
RapidSSL Wildcard worked perfectly in the test. When importing, we used the .P7B file, together with the .KEY file. - If you use an SSL for Sophos XG and have problems with the acceptance of your SSL certificate, we currently recommend RapidSSL: https://www.interssl.com/de/cart.php?gid=10