Error message: References from multiple groups

I added text from one document (with references from a Shared Group) to another document (citations from My Library). I was asked to import the missing references to My Library. Ok, I did that. But now I somehow got this error message when editing a citation: "This citation contains references from more than one group, which is not supported." How can this be fixed? Or: How can I change the source of the references without checking every single one? Thank you! (working with Win7, Mendeley Desktop 1.17.10, MS Office 365)