FortiGuardDDNS Failed

DDNS can basically provided by any provider.

Fortinet do provide their won DDNS Service which is included in FortiOS. This is called "FortiGuard DDNS". You can select one of their ddns domains there and set an interface to listen on. 

FortiOS also does support a number of third party ddns providers but those are unavailable at gui. If one needs one of those it has to be set up on cli.

I thus cannot say how to solve it but I encountered that several times that the FOrtiguard ddns simply did not work anymore.

I meanwhile tend to not use it anymore and use one of the also supported third party ones instead of it.