It seems like n8n crashed at some point and so the webhook did not get unregistered correctly. So does n8n check on start up if that webhook with the ID still exists.
That check fails. Which is correct and expected but apparently did the error-message format change (probably from the underlying library) and so it does not catch it correctly. That problem will be fixed with the next version.
So the best thing for you to fix that problem now would be to save the workflow under a new name and activate that one. As soon as you did that, delete the old one. All should then work fine.