Answer
Check for custom events or actions still referencing a BPA workflow that the upgrade itself removed — an upgrade can remove standard/example workflows without automatically cleaning up custom events or actions that still point at them, leaving a dangling reference that blocks enabling new custom events/actions in the same area; re-importing the missing workflow (for example, from a pre-upgrade environment or another instance that still has it) to restore the reference has resolved this exact class of post-upgrade error.