I had a very frustrating error and wanted to share the solution.

The error appeared when the username or email was checked for existent by ajax during registration:
Fatal error: Call to undefined method JDocumentRaw::getHeadData() in
/var/www/vhosts/lovda.nl/httpdocs/libraries/joomla/cache/cache.php on line 531

Solved by disabling the System - Cache:
Extension Manager-> manage -> System - Cache

I hope this helps someone