In fancyhdr v5 the number of arguments to \f@nch@head and \f@nch@foot has been changed. This resulted in an error with scrlayer-fancyhdr. This release of scrlayer-scrpage takes care to run \f@nch@head with the correct number of arguments. However, I'm not sure, that all of the arguments are functional as they should be. More tests would be useful.