Finally I have localized the problem, although I am really not sure what happens (due to a limited knowledge to all constructs in perl).
However, I localized the problem to be in the subroutine "send" in the email.pm file.
By replacing the line
my $header = "To: " . join ';', @{$self->{to}} . "\n";
with
my $header = "To: " . $self->{to} . "\n";
the problem is solved (at least in my case).
Yes, I know that my replacement does not take into account that $self-{to} might be an array as some comments in the code suggest. However as the emailNotify2 is build, the plugin does not use that option anyway...