When you have a birthday in a contact, then this birthday will appear in the calendar (if you select birthdays this is)
Now, if you delete that calendar entry it will create a sync problem
So eM Client doesn’t feed back the deleted birthday calendar entry into the address book contact. That’s a bit annoying and worth fixing one day.
Workaround: You actually need to delete the birthday in the contact.
That is probably because the birthday folder, which comes from the server, is read-only.
If you try and create an event in that folder it just disappears.
I understand the problem.
In my opinion, what should happen, is that either
a) eM doesn’t allow you to delete, ideally give a hint that this is read only
b) just overwrite at the next sync
Instead, eM is generating a sync conflict error, which is a bit disturbing and then requires you deal with it etc. For you it may be plain obvious, I had to spend some time to figure out