Silverstripe 3.x - All form submissions throw a 403 error - silverstripe

Question

I recently migrated a Silverstripe 3 site to a new server and now all form submissions on the site throw a 403 Forbidden (nginx) error.
I've read references of this symptom, but can find no leads which seem to apply to my situation as none of them change the behavior. However, my web host does suggest it could be a .htaccess issue.
My .htaccess file at the site root is as follows:
### SILVERSTRIPE START ###
<Files *.ss>
Order deny,allow
Deny from all
Allow from 127.0.0.1
</Files>
<Files web.config>
Order deny,allow
Deny from all
</Files>
# This denies access to all yml files, since developers might include sensitive
# information in them. See the docs for work-arounds to serve some yaml files
<Files ~ "\.ya?ml$">
Order allow,deny
Deny from all
</Files>
ErrorDocument 404 /assets/error-404.html
ErrorDocument 500 /assets/error-500.html
<IfModule mod_alias.c>
RedirectMatch 403 /silverstripe-cache(/|$)
RedirectMatch 403 /vendor(/|$)
RedirectMatch 403 /composer\.(json|lock)
</IfModule>
<IfModule mod_rewrite.c>
SetEnv HTTP_MOD_REWRITE On
RewriteEngine On
RewriteCond %{REQUEST_URI} ^(.*)$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_URI} !\.php$
RewriteRule .* framework/main.php?url=%1 [QSA]
RewriteCond %{REQUEST_URI} ^(.*)/framework/main.php$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule . %1/install.php? [R,L]
</IfModule>
### SILVERSTRIPE END ###

Answer 1

Have you checked your $allowed_actions property in the Controller.
In SilverStripe 3.1.0 you need to define $allowed_actions as a private static
See the following link for more info
http://doc.silverstripe.org/framework/en/changelogs/3.1.0 on Upgrading (info below)
Statics in custom Page classes need to be "private"
Requires action on every SilverStripe installation.
Typical error message: Access level to ErrorPage::$db must be public
Related to the configuration change described above, many statics in core are now marked with private visibility. While PHP allows making variables more visible (e.g. from "private" to "public"), it complains if you try to restrict visibility in subclasses. The core framework extends from the Page class in your own codebase (mysite/), which means you need to change those statics to private yourself. The same rules apply to controllers subclassd from Page_Controller.