Forticlient 5100 error connecting to our fortigate

Morning, we have an outside contractor that is getting -5100 Fortigate does not support dual stack when trying to connect.

We don't use ipv6 and don't have dual stack setup in any way. Our VPN is of course working perfectly for our 60 users.

I don't plan on changing anything major for them to connect but if there was some minor change I could do on my end to allow them to connect I would consider it if anybody has a suggestion.

I've done some googling and came up with this list:

• Its an issue with forticlient 6 and if they upgrade to 7 that will solve it.
• I've also seen posts suggesting the client has to disable ipv6 on their endpoint if they want to connect.
• Its DNS and they should change from URL to IP address.

Any of that sound right? They keep reaching out saying "fix this so we can connect" but they continually miss call times we have setup to work on this then just email again saying "fix it" so I haven't had a chance to even talk to them yet. Its very frustrating and I'm sure they will have their own IT people on their end I'll have to work through with any suggestions.

Thanks