After update from Joomla 3.5.0 (or previous version) to Joomla 3.5.1, you could experience problem with Rapid Contact and Hot Appointment module (based on Rapid Contact). This problem manifests as 500 (server error) page every time when user submits the form. We will explain how to fix this.

You should edit file /modules/mod_rapid_contact/mod_rapid_contact.php and find this part:

$mailSender->addReplyTo(array( $_POST["rp_email"], '' ));

Change this piece of code with this:

if(version_compare(JVERSION, '3.5', 'ge')) {
$mailSender->addReplyTo($_POST["rp_email"], $fromName);
else {
$mailSender->addReplyTo(array( $_POST["rp_email"], $fromName ));

If you are using dentist Joomla template, beside the mentioned file, you will need to make the same modification to the file /modules/mod_hot_appointment/mod_hot_appointment.php

Using FTP client, upload the modified file(s) to your server (overwrite existing file). After this, the forms based on Rapid Contact and Hot Appointment modules will work normally again.