Problem:
Users can´t login anymore into Nextcloud if your environment have more then <500 Nextcloud users.
Solution
source: Nextcloud Enterprise Univention App - Nextcloud
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.