For other people who have that similar issue and can't access the WPML String Translation to delete the translations with the textdomain "default" :
1) Install the plugin WP Downgrade (by Reisetiger) and activate it: https://wordpress.org/plugins/wp-downgrade/
2) Go to the plugin WP Downgrade setting page and set the roll-back to WordPress 6.2.2
3) Go to Dashboard > Update and reinstall WordPress 6.2.2
Now you should have access to WPML > String Translation.
4) WPML > String Translation > select the textdomain "default"
5) Select all the string within this "default" textdomain, delete them.
6) Go back to Plugins > Desactivate WP Downgrade
7) Go back to Dashboard > Update and install the last version of WordPress.
Hope this can help someone.
PS: to the support team, note that the latest WPML String Translation release 3.2.7 didn't resolve the initial issue. Rolling back to WP 6.2.2 to delete the string with "default" textdomain did the trick.