I was having the same issue as most of you all but mine was caused because my site uses an SSL and after PayPal process the payment it was trying to post to the validation page via http instead of https. Most browsers don't like doing that for security purposes, whiched caused the hang-up.
I set up my PayPal Manager as surferboy recommended with a few variations. For the urls I just removed the 'en'. I also included https into the urls, but that didn't seem to fix my issue.
It wasn't until I went into /modules/paypalusa/paypalusa.php on line 416 and replaced both instances of
$this->context->link->getModuleLink('paypalusa', 'validation')
with
https://www.[mysite].com/module/paypalusa/validation
paypalusa.php
After that my module started processing the orders correctly.
The link for your site may be a little different depending on how your site is setup.
I am actually using PayPal USA version 1.3 found at https://github.com/PrestaShop/PrestaShop-modules, although had to manually add the contents of backward_compatibility folder in paypalusa after downloading the master.zip.
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