Google has deprecated (killed off) Brand Accounts. If you had a brand account linked to your KPS account, there's a chance you can still access your videos with the following steps:
Go to account.google.com
Click Data & Personalization
Scroll down to Delete a Service and click that.
Click Delete a Google Service
You will be required to log in again.
Click the trash icon next to youtube and click confirm many times
Try watching videos now.
For more information on the backstory regarding Brand Accounts, see this AmpliFiedIT article