Webtop seems to just allow just appending the domain that is set in the Webtop app settings in cluster-admin without checking LDAP.
Nextcloud seems to check the LDAP mail field but it’s not filled with a mail address.
To enable login by mail address you need to enable “LDAP/AD Email Address” in the “LDAP/AD-Integration” in Nextcloud admistration settings.
To set mail addresses in LDAP/AD you could use LAM, see also ns8:applications:lam [NethServer & NethSecurity], as regards AD RSAT tools should work too.
Now you should be able to login to Nextcloud using the mail address.