TOTP 2fa prevents successful card/caldav logins on iphone/ipad

iPhone 6S iOS 12.1.1
iPad Air iOS 12.1.1
Nextcloud Server 15.0.0

While trying to create accounts on iphone and ipad to sync calendar and contacts, authentication kept failing.
I disabled TOTP 2fa authentication, then I could successfully sync the accounts on the devices with Nextcloud server.

I stumbled about this problem today too. If TOTP is activated there are error messages on the iphone about a wrong password entered. As soon as I deactivated TOTP the sync is successful and I could see an entry in “security” and “sessions” . If I set a password for this “app” I get the same error message on my iphone.

Is there a hint where to look for a solution or a configuration?