The new location of the Nextcloud client PPA is ppa:nextcloud-devs/client. To start using it, issue the following commands:
sudo add-apt-repository ppa:nextcloud-devs/client
sudo apt-get update
You can remove the old one by:
sudo add-apt-repository -r ppa:ivaradi/nextcloud-client-ppa
I am going to update only the new PPA and will delete the old one in some time.