This error is caused by an incomplete PHP configuration. Both SOAP and XML extensions need to be enabled in PHP in order for the UI to work.
On linux, make sure the following packages are installed (PHP SOAP, PHP XML, PHP DOM). You can install them by typing in the following commands:
yum install php-xml
yum install php-soap
yum install php-dom
On Windows, make sure the php.ini file has the following extensions enabled in the php.ini file and then restart Apache: