CalDav Sync Problem with mobile app

Good Day I have been trying all day to sync my calendar to the mobile app. The error reads Cal/Dev Card/Dev an attempt to connect to … This could be caused by temporary server availability or incorrect settings.

Please help

What account is it ?, and what are your CalDav & CardDav server account urls ?

Also did you add in your account via using the QR Export code via eM Client for desktop ?