This might be due to a misconfigured app proxy URL. The proxy URL is user-configurable and in rare cases some users might change this URL. But the app only works with a specific URL and redirects aren't possible.
Solution
Go to Settings > Apps and sales channels and then click B2B Handsfree.
Check that the Proxy URL is https://STOREDOMAIN/apps/b2b-customer-register. STOREDOMAIN should be the actual store domain.