Just fyi, I’m not the author of this plugin, but the authors (M&S Consulting) do have a paid support deal for $10. One of you might consider doing that to have them fix this & release a new version.
That said, you can probably hack in a fix with just a few changes in email-before-download.php. I have not tried this because I don’t have a 4.4 setup, so you guys will have to try this & see if it works…
So what I think needs to happen is that you need to find the 5 lines that are using “WP_PLUGIN_URL” (which is a wordpress internal variable or constant I believe) and change it to “plugins_url()” and I think that will do it. The double-quotes are not included, I just put them there to make it clear what is changing.
Before you change email-before-download.php, be sure and make a copy of it and put it in some other directory, just in case. So this is basically a straight text replace of that exact string — don’t change anything else around it like spacing before it for that period that comes right after it, you just want to make WP_PLUGIN_URL become plugins_url() and leave everything else exactly the same.
There should be 5 occurrences of that I think. I believe it will be lines 531, 588, 844, 853, and 904.
After making the changes, save it. Use your browser and go to your page that has your email before download form on it and hold down the shift key & click reload (or refresh) to make sure you get a new copy (rather than a cached copy) and then retest and see if you have the same problem (or a new problem). If it doesn’t work, post the URL it gives you back in here like you did initially.
My guess is that this problem will only affect WP 4.4 installations that are using https … people using http may not see the issue (or they may, I’m just guessing really).