I'm having the same problem. Brief mode has worked fine on all my devices for months - this morning they all insist on saying "OK". Tried the usual rebooting, turning off and on but no luck. Interestingly, my iPhone is showing that the Alexa app was updated today, maybe this is the cause of the problem.
I have just spoken with Amazon customer services and after a bit of prompting they admitted that they had just released new software and "Brief Mode" no longer works. They said engineers are working on it. It is a pity that they didn't test their software before releasing it!
This started for me last night too, on all my devices. Hope it's fixed soon, it's already driving me mad. I've only had these devices a couple of months and I'm spending far too much time in this forum trying to fix annoyances that happen despite following instructions to the letter.
I’m glad to hear that this is going to be fixed. It’s pretty rubbish to roll out an update that breaks functionality like this. I was super-unimpressed by the poor response I got from Customer Support when I report this, too. I expect more from Amazon. The Echo is a great device but even the best technology can be spoiled by such slipshod support