Nextcloud has some kind of flood protection - if the pw was entered wrong for a few times, it does not accept requests for a period of time (minutes I guess).
Another thing : Did you enable app specific passwords and two step verification? If yes, you need this password to login and not the one you use to login to Nextcloud!
I’m having the same problem but the app password was entered correctly the first time. But CalDav says it can’t find any DAV service .
See: link Link