Cookie Error is a bug in the WP Core

It is not a WordPress core bug. It’s usually an issue with your hosting: https://www.ads-software.com/support/topic/cookies-are-blocked-or-not-supported-since-update-to-371/page/3#post-4900877

There seem to be many reports that are not in the “usually” category. Would you call this a “feature”?

There are hundreds of posts about this behavior, which seems to be triggered by routine use. I think that’s a bug.

Would you call this a “feature”?

No. As I said, it’s a server configuration issue.

I am not going to be of much help getting your server or Core code sorted out, rvbinder.

I remain confident that the use of this plugin has revealed something amiss with your server config, and not with the plugin in and of itself.

Here’s an update.

1) I was able to determine the cause of the cookie error message and correct it. The problem occurred immediately after I added a new site in Multisite, which I have configured in “sub-domain” mode, but in which each site is presented as a unique full domain (a.com, b.com – not a.c.com, b.c.com) Each such domain is “parked” at my ISP. The last step in establishing a new domain is to obtain the well-hidden site id, then enter this into the panel presented by WordPress MU Domain Mapping plugin. I’d simply forgotten to do this last step. When I backtracked and did that, the login worked normally.

2) So, I’ll retract my characterization of this as a “bug” in the core. However, it certainly seems to me that the existing implementation is flawed in its handling of this situation and could provide better diagnostic information.

@sader I started this thread to let you know that the problem was not related to your plugin.

“When I backtracked” …

Backup, go backwards, try and try again. A good lesson.