My hoster told me that this php-xml extension is already active by default.
So this is the issue why I am getting errors while doing the import with WP 5.5?
You may want to relay this part of your error_log to them:
[Thu Aug 20 18:36:51.460986 2020] [-:error] stderr: PHP Warning: DOM support is not enabled in ./wp-content/plugins/gp-premium/sites/libs/wxr-importer/WXRImporter.php on line 392, referer ./wp-admin/themes.php?page=generatepress-site-library