Since 2.1 I have an issue where I have to log in every time I open a page in the Admin section.
I log into the dashboard. I click on All Posts. I have to log in. I right click on the Create New button to create 3 new posts. Each blank new post wants me to log in again.
Is this an option I can turn off, a bug, or something peculiar to me?
Not happening on any of my sites.
Have you done the basics such as making a full backup, then disabling all your plugins and changing to a default theme?
We’ve had 2.1 running as a nightly for quite a while, and no-one reported anything like this. I can’t reproduce it on my own sites.
Anytime I have seen that happen, it was due to how ssl was being handled (by the proxy server). I do not know your configuration, but that is someplace to start maybe.
@EliteStarServices’s comment reminds me that this sort of thing can happen if you have a mismatch somewhere in how you write the URL for your site.
In other words, https://mysite.com/ is not the same as https://www.mysite.com/ and so you’ll need to keep logging in to go from one to the other. That will also be true if some of your pages are on https and some are on just http.
I think I have located the source of the issue, and it is not what I thought it was. The problem is as I described but I have been looking in the wrong place for the solution.
I use Vivaldi on Windows 10 as my browser and it is there that the issue first appeared. I have just tried it on Firefox and there everything works as expected. I then tried it on Chrome (Vivaldi is Chromium based) and all work properly there too.
So I now think that the problem lies in a recent update to Vivaldi, which has either introduced a new security feature (which I need to find and turn off) or a bug.
Many thanks for your suggestions, all of which helped me identify where the real problem is!
And then…
It turns out to be more complex or more baffling. Everything I wrote above in both posts remains true but, as I have said in another thread, I maintain two ClassicPress sites. Both are on the same server and it turns out that the second site behaves perfectly in Vivaldi.
The only difference between the two sites is that the one that is not working properly is at root level and the second site sits in a subdomain. I assume that any issues with SSL should affect both of them.
I have backed up the first site (actually I back it up nightly with Updraft Plus), switched to the default theme, and disabled all the plugins. None of that has any effect whatsoever. I have checked the url and that is okay.
What baffles me is that I have done nothing to the site (other than post content) for several months and this problem only appeared a few days ago, and only apparently in Vivaldi, while Vivaldi continues to treat the other site as it should.
I simply don’t know where to look to find what has gone wrong.
That behaviour is certainly strange. If it’s only happening in Vivaldi, then it sounds like something to be reported to the Vivaldi devs.
But, just in case, have you checked both sites’ URLs in their respective admin Settings pages? Both boxes on that page need to match each other, but they also need to match the format of the other site.
I have just checked, and yes both boxes and both sites match each other and are in the same format.
In doing this I realised that what seems to be actually happening is that ANY attempt to do anything that in effect loads or refreshes the page causes a login request. So If I simply reload any page in the admin section by choosing “Reload” from Vivaldi’s context menu then that triggers a login demand.
But only on one of the two sites 
This is very likely a browser storage issue, try flipping Vivaldi into the equivalent of Private browsing to see if that fixes the issue on the affected site. If it does, they you probably need to clear the browser cache.
SOLVED
I have an answer from a Vivaldi ambassador on the Vivaldi forum which I will quote in full since, although it makes clear that the issue lies in Vivaldi, it does relate to Classicpress and its use of cookies. The ambassador installed Classicpress on a test site to experiment and found this:
I was able to reproduce the problem on my test site.
Found a related bug report: VB-107557
Looks like it only happens if the link is opened from the context menu.
Technically, cookies with SameSite=Strict are not sent in new tabs opened from the context menu. This is not limited to ClassicPress.
I will ask the devs to prioritize this one, it’s a nasty one.
For now, don’t use the context menu to open links.
Use Ctrl-click or Shift-click - it’s also faster 
That’s why I couldn’t reproduce this, I always use Ctrl/Shift-click.
You did say “right-click on a button” but I wrongly assumed it would behave the same… 
It also looks like WordPress does not use SameSite=Strict cookies.