Hrm- first, excellent job of debugging on your end.
Second- since it’s a fresh install, the cookie domain/path/prefix were likely blank already, so you’re not affecting anything there.
The big changes are the ones affecting the IP check (secure forms basically checks that the ip that loads the page is the same as the one that submits it, require IP simply requires a valid ip exist to submit).
None of those IP related settings are set according to your overrides by default. So it’s pretty likely IP is related to the problem. If your IPS provider has a rapidly revolving IP? It can cause those security checks to fail. Does that sound like what you’re experiencing?
Ah- hrm. Or- you’re on 2.5.1- which won’t recognize an ipv6 format as a valid ip. If that’s the case? Updating to 2.5.2 should solve the problem- it’s ipv6 compatible. And thus a good idea to update to it in any case.
So short answer- I strongly suspect your ip is either being seen as invalid or it’s rotating very quickly. Updating will solve the first problem- with the second problem, you’d need to leave your settings adjusted to accommodate the rapidly changing ip. Make sense?