Creating a new topic leaves a white page

Hi there

I just downloaded an Installed a new version of phpBB. I have php7 on my server thats why I tired the recent release candidate.

However, the setup worked after I realized that I have to install php7.0-mbstring. Now I got stuck in a different problem:

I erased all default topics and forums, and created a new one. Now I tried to post my first new topic. When I press the Submit button I get a white page. The php error log shows the following message:

PHP Fatal error: Uncaught Error: Class 'DOMDocument' not found in /var/www/forum/vendor/s9e/text-formatter/src/Configurator.php:1733\nStack trace:\n#0 /var/www/forum/vendor/s9e/text-formatter/src/Configurator.php(1353): s9e\\TextFormatter\\Configurator\\Helpers\\TemplateHelper::loadTemplate('<br/>')\n#1 /var/www/forum/vendor/s9e/text-formatter/src/Configurator.php(6209): s9e\\TextFormatter\\Configurator\\Helpers\\TemplateForensics->_construct('<br/>')\n#2 /var/www/forum/vendor/s9e/text-formatter/src/Configurator.php(8723): s9e\\TextFormatter\\Configurator\\RulesGenerators\\EnforceContentModels->_construct()\n#3 /var/www/forum/vendor/s9e/text-formatter/src/Configurator.php(8371): s9e\\TextFormatter\\Configurator\\Collections\\RulesGeneratorList->normalizeValue('EnforceContentM...')\n#4 /var/www/forum/vendor/s9e/text-formatter/src/Configurator.php(4765): s9e\\TextFormatter\\Configurator\\Collections\\NormalizedList->append('EnforceContentM...')\n#5 /var/www/forum/vendor/s9e/text-formatter/src/Configurator.php(47): s9e\\TextFormatter\\Configurator
RulesGe in /var/www/forum/vendor/s9e/text-formatter/src/Configurator.php on line 1733, referer: https://forum.***.de/posting.php?mode=post&f=3

If there are any more information you need, feel free to ask.

Best jesaa