Hello @Greystoke,
Thank you for reaching out to the Quicken Community, though I do apologize that you are experiencing this issue.
First, I suggest that you sign out of your data file completely and then sign back in. In doing so, you will refresh the registration token for your Online Connected Services. Please follow the steps below in order to do so.
After signing out and then back in, please attempt to add your Charles Schwab account(s) again.
Please let me know how this goes!
Thanks for coming back to update us.
I am happy to hear that we were able to reach a resolution!
Please don't hesitate to reach out with any further questions or concerns.