The solution was that FOnlineIdentityXYZ is not a UObject and I can therefore not use BindUObject, but must use BindRaw instead. In addition I am now using
TSharedPtr<FString> shareableToken = MakeShareable<FString>(new FString(AccountCredentials.Token));
… and pass that to BindRaw to ensure the FString isn’t dealocated when it goes out of scope.
May this help someone else out there!