Hi,
a few other users reported the same and I have now found out that this is a new feature of Google Calendar that was announced here:
I.e. Google has now removed the separate birthdays calendar and instead put the birthdays into the main calendar. I think it would have been better to keep it separate but Google has changed it this way now.
Anyway, they have also added a field "eventType" to see which events are birthdays and which are normal events. I will try to create a new update soon that uses this field to allow you to hide the birthdays or to display them with a different color so that it looks similar as before.
Please don't hesitate to contact me again if you have further questions!
Kind regards
Dominique