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SimpleSAML\Error\Error: UNHANDLEDEXCEPTION

Backtrace:
1 www/_include.php:20 (SimpleSAML_exception_handler)
0 [builtin] (N/A)
Caused by: Error: Class 'DOMDocument' not found
Backtrace:
4 vendor/simplesamlphp/saml2/src/SAML2/DOMDocumentFactory.php:109 (SAML2\DOMDocumentFactory::create)
3 vendor/simplesamlphp/saml2/src/SAML2/XML/md/EntityDescriptor.php:461 (SAML2\XML\md\EntityDescriptor::toXML)
2 lib/SimpleSAML/Metadata/SAMLBuilder.php:115 (SimpleSAML\Metadata\SAMLBuilder::getEntityDescriptor)
1 lib/SimpleSAML/Metadata/SAMLBuilder.php:135 (SimpleSAML\Metadata\SAMLBuilder::getEntityDescriptorText)
0 www/saml2/idp/metadata.php:215 (N/A)