Thank you for your post.
We are currently testing this very plugin (Pie Register) for our own side, and we ran into HTTP error 500 on 2 of our test servers. Your post points my developer’s team into a right direction.
The solution offered (remark out line 1188 in “pie-register.php” is solving 95% of the problem. However, there is one scenario, when this fix doesn’t work:
If you are already logged in into the site in one browser session, and trying to register in another browser session, you still will get a blank page on HTTP 500 error depending on the browser you use.
By any chance, do you have a solution for this problem?