False positive link warning

Hi,

we (a Scandinavian ESP) have a customer, who in turn has a recipient that is using Fortiguard. The links in the mails sent from us/our customer are blocked by fortiguard because of "DLG_FLAGS_INVALID_CA". 

The link domain in question is https://tr.aonetrk.com and we can't find anything wrong with the certificate?

Thanks in advance.