I don't know if this is the answer to your question, but I'll mention it, since you are not showing any code. Are you passing your %args in correctly? There is a awkward way of doing it.

my %ssh_options = (
 StrctHostKeyChecking => "no"
 
);

my %args = (
 username  => $user,
 password  => $pass,
 debug     => 1,
 ssh_args  => (options => \%ssh_options)
);

my $sftp = Net::SFTP->new($host, %args)
[download]

I have seen that same exact error using Net::SSH::Perl , and it is fixed by using "protocol => 2";

Maybe the server you are connecting to is using protocol 2, but you are not setting the protocol=>2 correctly for SFTP to pick it up?