I get "Internal server error 500" or just blank page when try to submit a form

This is very insidious error because it doesn't tell us what is the error actually, just "an error". However there are several common

Most probably the problem will be solved by one of the following solutions:

1. Try to remove write permissions from the "forms" folder (set chmod 775 or 755 - this must help).
2. If you save submissions to the database, delete your current log template and create new. It should be done every time you change the set of form fields.
3. Check email template headers. Maker sure that each header has either valid email, or existing email field variable (e.g.: [re_email]).
4. Some mail servers restrict sending of carbon copies of email (CC and BCC). Try to remove these headers from email templates.