Start with debug flow to make sure it is indeed malfunction and not an explicit block on some security feature.
diagnose debug flow filter addr <IP of testing PC>
diagnose debug flow show function-name enable
diagnose debug flow trace start
diagnose debug enable
If the above tells nothing and you see all passes as Allowed, try making an exception for this website - 1st in Web filtering in Static URL filter (put action Exempt/Allow), if no help - add to SSL Exception in profile (this exempts this website from SSL inspection completely). If this helps (most probably will) the next would be to open ticket with Fortinet TAC and investigate further what Fortigate does not line about this particular website.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.