I upgraded Expression engine from 1.6.8 to 2.5.2 and in the process upgraded Freeform from version 2.6.0 to version 4.0.5.
FreeForm Forms now no longer return email, and they do not return errors. They go to the appropriate response forms that were defined for them in the earlier (and index.php is included in the return URL).
A complete an total enigmatic failure of FreeForm, with NO error messages.
The standard Expression Engine member module registration email works. email_test.php returns email (i.e., email is configured on the site/server). And PHP mail scripts work.
Only FreeForm fails to send email, with no error messages.
I am upgrading a cloned site at a different URL than the original site. Once issues are resolved, we intend to switch it back to the older URL. There is newer PHP and newer MySQL, and a newer installation of WordPress on the site.
FreeForm seems to be the only thing that does not work. And it is preventing upgrade of the site (going live).