See stacktrace coming from api.php:
<br /> <b>Fatal error</b>: Uncaught Error: Call to undefined function normalizer_normalize() in /var/www/html/quickstatements/public_html/quickstatements.php:578 Stack trace: #0 /var/www/html/quickstatements/public_html/quickstatements.php(568): QuickStatements->compareDatavalue(Object(stdClass), Object(stdClass)) #1 /var/www/html/quickstatements/public_html/quickstatements.php(1135): QuickStatements->getStatementID(Object(stdClass)) #2 /var/www/html/quickstatements/public_html/api.php(235): QuickStatements->runSingleCommand(Object(stdClass)) #3 {main} thrown in <b>/var/www/html/quickstatements/public_html/quickstatements.php</b> on line <b>578</b><br />
Cause seems to be that normalizer_normalize is missing. Which looks like it might be supposed to be a php builtin: https://www.php.net/manual/en/normalizer.normalize.php