Change PBI API version due to "The expected type 'Collection(Edm.String)[Nullable=False]'" error

Hello everybody,

I try to get data of a SharePoint online list but I get this error:

'group1' is a multiple selection listbox that can be blank! If I remove this filed, the list connects perfectly!

It seems  that using API 14 instead of 15 resolves the problem but I'm pretty new to PBI and don't know how to change it! 

I would be grateful if somebody could help me.