After migrating this install from PHP 7.4 to PHP 8.1 the form stopped working
after adding an extra field with a simple validation to make sure the end user
solved a simple written out sum the form stopped working. First with a general:
Oops!
It looks like something went wrong and an error has occurred.
A notification has been added to the admin log.

and this error in the admin log:
Unable to load template 'module_file_tpl:SmartForms;error.tpl'

After I add a error.tpl file in the modules' template dir I'm getting this error
on the frontend:
Sorry, something has gone wrong. We could not find a valid unique request id in
the posted data

which seems a bit like the bug reported here:
http://dev.cmsmadesimple.org/bug/view/12028
Since this is also only happening when nog logged into the admin. If logged in
to the admin the forms submit beautifully.

The php error log only displays this deprecated message:
PHP message: PHP Deprecated:  Constant FILTER_SANITIZE_STRING is deprecated in
/home/username/domains/domain.com/public_html/modules/SmartForms/lib/class.FormResponse.php
on line 55'