I've been having this problem since MR6 where I cannot save any IPSec policy at all.
There is not error logs, it just shows VPN policy "name" could not be created.
I am on 17.1.1 MR-1 now, it is still the same.
Try restoring to default, and even deploying a new XG, it is still the same.
What am I missing?
I had the same issue (same firmware etc), it was resolved by using Firefox (was kindly helped by Sophos Support).