It's very likely that the cronjob change is related to the issue. That script was meant to be called as a webcall and not as a php system call.
You may ask your server for a way to execute it as a web call or you may create your own php script to do it for you. This document might be of help:
https://davidwalsh.name/curl-download
Also there are a few services out there tha can help you if you cannot make it work in your hosting:
https://cron-job.org/en/
And finally you may execute the call to that file from other hosting company/account on your portfolio and it will work

.
Even calling it from your webbrowser or your computer should work.
If you are sure you are not going to update soon and cannot use a web call in any way for this, you can check the code for the sendreminders.php function and check around line 433. There you may find something like this:
$regex = "#{LINK}(.*?){/LINK}#s";
preg_match($regex, $message, $matches);
if (count($matches) == 2)
{
$Itemid = JRequest::getInt("Itemid");
list($year, $month, $day) = JEVHelper::getYMD();
$link = $row->viewDetailLink($year, $month, $day, true, $Itemid);
if (strpos($link, "/") !== 0)
{
$link = "/" . $link;
}
if (strpos($link, "plugins/jevents/rsvppro") !== false)
{
$link = str_replace("plugins/jevents/rsvppro/", "", $link);
}
if (strpos($link, "plugins/jevents/jevrsvppro/rsvppro") !== false)
{
$link = str_replace("plugins/jevents/jevrsvppro/rsvppro/", "", $link);
}
if (strpos($link, "plugins/jevents") !== false)
{
$link = str_replace("plugins/jevents/", "", $link);
}
which you should be able to modify and adapt to your needs.
Sorry we cannot provide further assistance!!