Dear Stefano Lissa,
After installing Newsletter 3.0.2 on WP 3.4.2 I encountered fatal bug.
/plugin.php line 843
eval('function ' . $name . '(){global $newsletter;require "' . $file . '";}');
Here You have a string variable wrapped by doublequotes inside Your evaluating code. This means, a path like
D:\n_as_first_letter/newsletter
Is parsed as
D:
_as_first_letter/newsletter
And raises a Fatal error on a later request command. Note that mixing up backslashes and forward slashes is not pretty, but unavoidable on a Windows server.
A quick Fix would be, to replace the doublequotes by escaped single quotes:
eval('function ' . $name . '(){global $newsletter;require \'' . $file . '\';}');