Sharepoint connection fails credentialing when adding a custom column in Power Query

Good morning - 

I have a connection to a Sharepoint list in Power BI. It's been working great, however.. when I try to add a custom column, Power Query and PowerBI both ask me for my Sharepoint credentials. I choose the subsite level, and it tells me that the user is not signed in. I sign in, and it tells me that "We couldn't authenticate with the credentials provided. Please try again". 

Does anyone know what's going on with this, and how to get around it? I was able to add a simple subtraction column in Power Query, but this is an if function grouping this created column:

if DurationValue > 360 then "360 + days"
else if DurationValue >= 90 and DurationValue <= 359 then "90 - 359 days"
else if DurationValue >= 60 and DurationValue <= 89 then "60 - 89 days"
else if DurationValue >= 30 and DurationValue <= 59 then "30 - 59 days"
else "Less than 30 days"

Thank you for any help!