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I've been getting outlook prompts constantly asking for my username and password. I read another thread and think I have found a solution for my issue. I have a password manager that populates the username and password. I had always thought I messed up my password, but just found out it was my username!
HERE IS MY SOLUTION": Do not enter the @cox.net part of your email address, but DO enter ONLY the first part of your email address as your username. I thought I used a nickname as my username - WRONG. The next time the prompt popped-up, I entered only the part of my email address before the "@", along with my password. That was more than an hour ago and I haven't been prompted again!! I haven't changed my password in months, so there is no need to do that.
I hope this solves the pesky pop-ups issue for good!
Still working for me - no more pop-ups since I corrected my username.