Keep Coding

I did a fresh install of 16.2 and when I try to register a new user I get the error below. I've re-extracted the zip and reinstalled the database, but I get the same error.
Tikiwiki is in a public_html/tiki folder and I have made no adjustments to it.
Any help would be appreciated.
Here's the error:
Fatal error: Uncaught exception 'Zend\Math\Exception\RuntimeException' with message 'The RandomLib fallback pseudorandom generator is not installed. Please install it to support secure random numbers' in /home/ghsfhaco/public_html/tiki/vendor/zendframework/zend-math/src/Rand.php:75 Stack trace: #0 /home/ghsfhaco/public_html/tiki/vendor/zendframework/zend-math/src/Rand.php(60): Zend\Math\Rand
getAlternativeGenerator() #1 /home/ghsfhaco/public_html/tiki/vendor/zendframework/zend-captcha/src/AbstractWord.php(354): Zend\Math\Rand
getBytes(32) #2 /home/ghsfhaco/public_html/tiki/vendor/zendframework/zend-captcha/src/AbstractWord.php(340): Zend\Captcha\AbstractWord->generateRandomId() #3 /home/ghsfhaco/public_html/tiki/vendor/zendframework/zend-captcha/src/Image.php(447): Zend\Captcha\AbstractWord->generate() #4 /home/ghsfhaco/public_html/tiki/lib/captcha/captchalib.php(150): Zend\Captcha\Image->generate() #5 /home/ghsfhaco/public_html/tiki/vendor/smarty/smarty/libs/sysplugins/smarty_internal_templatebase.php(171) : eval()'d c in /home/ghsfhaco/public_html/tiki/vendor/zendframework/zend-math/src/Rand.php on line 75

This also caught us out the other day and it's due to an older version of PHP 5 which hasn't got the mcrypt extension enabled, and we fixed it recently, so there are a few options for you I think:
Upgrade to php 7 (probably not an option unless it's your own server)
Enable php-mcrypt for you php (also probably not possible)
Try again with a recent "daily build" 16.x tarball from dev.tiki.org
Or enable one of the other CAPTCHA methods on Control panels->Security->Spam Protection, such as ReCaptcha or CAPTCHA Questions
HTH