Each time I'm trying submit the form, it shows only blank page or 500 error.

There could be a few solutions for this issue:

1) Check permissions on folder "forms" - there is usually enough permissions 755. But you can also try 644, if error is still there.

2) You created a form with log template, after you made changes in form - and log template became outdated. Please remove that log template, and create a new one.

3) Check permissions on file "formprocessorpro.php" - 655 or 755 is the best choice.