Contacts disappeared and CarDav error appeared

Hello, my grandma recently upgraded to Windows 10 and put EMClient on the PC. She had it before on her Win 7 PC. At first, the contacts were there and everything was great, but after about a week, she lost all her contacts and an error started appearing. unnamed

She uses a @frontiernet.net address, which is Yahoo, I think.

She is on the most recent version (no updates available) - 8.0.3318 (1a4785e)