Not sure if this has been addressed.

Happens in FFA and Alliance for sure.

When you click to declare peace link it redirects you to the portal page. If you redirect and try again it works.

I notice it's also like this.
When displayed in war room as:
Country Name #1 <declare peace link> redirect to portal

Displayed like this:
Country Name #1
<declare peace link> actually declares peace.

Ahh yes I used to be really annoyed by this especially when sending my 5th PS on a target Im about to declare peace with. Then you get locked out of the war room so you cant declare peace at all lol.

Iirc the new UI works differently for this, I believe you can declare peace on a target even after PSing 5x. So if this is what bothers you I recommend logging in with the new UI to declare peace and then return to old faithful again. :)