Apparently there are no known cases of force:inputField working correctly. @Fab asked [this][1] question 9 months ago, and noone seems to have been able to get it working. I guess for now the best option is, like @Mohith suggested [here][2] and @Jenny commented above, to create your own lookup component. 

https://developer.salesforce.com/blogs/developer-relations/2015/06/salesforce-lightning-inputlookup-missing-component.html

I hope this will be fixed and working asap, as it's pretty hard to have users create sObjects without it.

EDIT

As @Peter posted on the thread linked above, there is currently a limitation in force:inputField's that you are required to add a default attribute like so:

    default="{ sobjectType: 'Account' }"


  [1]: https://salesforce.stackexchange.com/questions/64580/lightning-how-to-use-forceinputfield
  [2]: https://salesforce.stackexchange.com/questions/92699/how-to-get-look-up-field-onto-a-lightning-component-in-salesforce1