CalDAV for stopped working out of the blue

Been using the free version of eM for several months.  Now, out of the blue, when I launch eM I get a dialogue saying “Password required for CalDAV” with places to enter the user name and password.  Those fields are filled in with the correct info.  Even if I re-type in the correct info and press “OK” the error box closes, the OPERATIONS window opens for a split second and then it closes and the error box re-opens.  All the credentials are entered correctly.  Nothing has changed with my account.  I haven’t changed anything in eM.  It’s just stopped working.  Ideas?  Solutions?

Follow up - tried deleting and re-creating the CalDAV account.  Same problem persists.

Hello,Is it possible that you enabled two-factor authentication on your iCloud account? If so, you will need to generate an app-specific password for eM Client. If not, can you check your antivirus/firewall and add an exception for eM Client?