On December 6th 2024 some Google accounts stopped showing Birthday calendar. Unfortunately, the calendar is fully missing from the Calendar data we receive which is why the eM Client app cannot show it.
Birthday calendar is a special type of calendar so there is unfortunately no easy way to share it from the web interface and use it as an Internet calendar.
So far we noticed that Google accounts set to other languages (such as Czech) on the Google server are not affected as often as those set in English.

At this point, we can only wait for Google to resolve this strange error.

Other apps that use Google Calendar via CalDAV protocol (and not via Calendar API as eM Client) are seeing Birthday events inside the main calendar instead of a dedicated Birthday calendar.