Problem:
New created users can´t login to nextcloud, but the login to UMC works.
Solution
Source: Nextcloud Enterprise Univention App - Nextcloud
Nextcloud change the user model for the product:
For very small installations (<500 users) this is 100% free, though without any service or guarantees. For slightly larger installations (>500 users) we offer Nextcloud Enterprise, with support SLA, security information and all benefits that come with our enterprise product.
You can use to check the count, of the nextcloud users in you LDAP
univention-ldapsearch -LLL '(&(nextcloudEnabled=1)(uid=*))' | grep dn: | wc -l