If you are trying to login as a Vendor, maybe after receiving an email and following the registration link, whilst still logged in as a Seller in the Shopify app in the same browser, the Marketcube.io platform will redirect you to Shopify.
In order to avoid this please open a new browser or if using the same browser, open a new Incognito or Private window by going to the browsers menu:
File > New Incognito Window
Please note: If your email is still open in the same browser as Shopify then you can copy the link supplied in the email and paste it into the new Incognito window or Browser.
This process can also be used if you are trying to run the Seller and Vendor accounts together, maybe for testing purposes.
This is a limitation we currently have with Shopify, however based on user feedback we've added in an alert to inform the users about this behaviour.