Dialog for fileassociation appears after upgrade to office 2016

as the title sais, we have this dialog poping up on starting office-Progs after upgrading to Office 2016 from 2013 on Windows 7.

besides i guess this is because there is OpenOffice installed in parallel - but not sure

The standard Office-filetypes are associated with Office, but some OpenOffice-filetypes, which should stay associated with OO

how can i depress the dialog from users

is there a registry-setting or similar?