I am testing OsClass before migrating my actual listings system and I am try all functions ...

At the moment, I switched to "User must validate their listing".
Nice. The listing is not viewable. OK
The user receive an email with a validation link. OK
When cliking (or copy/past) the link, the listing is not validated and remains "inactive" in admin.

Any idea ?

That does not sound like ok.
Best to send such link and backoffice access.
Do you get successful message as well?

I did not have successfull message yesterday when I posted first this issue.
I discovered that this issue is linked with email format problem in PHP 8

This morning, I switched the server to PHP 7.4 and email format is good, and validation link is OK (message OK, listing appears normally)

I think there is an issue with PHP 8 ... my server (cPanel) or the script ?
I can help for tests ...

Hard to say what cause issues, as for mails external library is used.