I posted this before for a coworker and got no response... now I'm having the same issue, and I'd really like to resolve it. I have no problem customizing a list in InfoPath on my work laptop, but the past couple of days I've had to use my personal laptop. I open the list in classic view and click on customize in InfoPath from the ribbon. I'm prompted to authenticate, and my O365 credentials don't work. I've also tried opening InfoPath first, then connecting to the list, but again I'm prompted for my O365 credentials and they don't work. HELP!
Curious what happens when you try to authenticate with Office 365 - what's the error message? I'm leaning towards it being something with your IT Department and how they manage "Trusted Sites". If it works on a personal laptop it definitely feels like an Internet Explorer browser security change that has happened.