I am working on creating a Nintex Form in O365 and running into an issue. We have multiple list we store on a seperate site collection and these list are referenced on multiple forms. In Nintex for SharePoint 2013 you were able to use a "List Lookup" control to reference a list on a seperate site collection. Is this functionality not available in Nintex for O365? I have tried using a relative and full URL's in the field, but receive the same error. Any help or guidance is appreciated!
Unfortunately as the warning message states: "Loading the available lists failed. Please check the URL you have provided is a valid URL within the current site collection."
So yes, the lookup control does not allow you to get values from a different site collection. The field follows the native SharePoint lookup field functionality, where you cannot lookup to another site collection either.