In order to register with a guest account you need to set a password on the provided registration page. The link to that page (e.g. http://mycompany.tld/oc/index.php/apps/e2eeshare/register/oEOsFpFZVD55HmT4MtKl4/guest@example.com) is sent by email after having shared a folder with a guest for the first time. .
For Registration you just have to enter and confirm the password.
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