I switched to eM Client, but it failed using my CalDav calendar. It added me as an attendee for all shared calendar events. (For example birthdays)
As long as I did not change anything it was luckily only displayed locally like this. Also it displayed invites on all these events since it thinks I am an attendee I had to respond. Responding made me attendee online as well which “corrupted” it online aswell.
I exported the event from my online client and from eM Client and it clearly shows the problem.
I put it into attachment.
This error happens with version 10.4.4209.0 and 10.3.2619, but not with 9.2.2280.
That is why I am now running 9.2.2280 and enjoying it.
Of course I would like to update at some point 
On the right you see the correct one from online client, on the left you see the on eM Client creates.
Thank you for the great product! Hope you can fix it. Happy to further help!