QB Issue Resolution:

Some apps do use SSO (Single Sign On), but not a lot.  We make this available to all of our App Developers, and they can choose to use it or not.  So while it is available for all apps, not all apps use SSO.

Resolution for Issue 'Online Bill Pay' available: Yes (Solved).
Source: Intuit Community forum.

Online Bill Pay: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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