CalDav and CardDav with Synology NAS not working

I have a Synology NAS with DSM 6.1.2 running WebDAV, CalDav and CardDav Server.
I sync my contacts with Thunderbird CardBook and my Calendar with the Thunderbird Calendar. The same works with the Windows 10 Contacts and Kalendar cients.
Sync with my phone works as well.
So my assumption is, that my Server side config is right.
But when I use eM Client it always asks for a Username / Password as authentication is required. I understand that, and type in the right username and Password, but it is not accepted. It keeps asking and asking forever until I hit the cancel button.
Is this something you can fix?
It turns out that I really like the Client, but this is a real KO criteria for me. So I was wondering if I should Keep the Client and hope for a fix, or if I should uninstall and look for soething else.

Thanks for this great Client, hope you can help me here.

I too have struggled with this, first with Outlook on and off for years, then eMC until I realized the login credentials for the cal/cardav fields need to be your APPLE credentials , not email credentials. 

If you’re already doing that, then you need to go one step further and generate the new “third party app specific” password Apple is now enforcing since about mid-June. I just did this but don’t have time now to get into it. If you  browse these forums with a search key like “App specific password” and/or go here  https://support.apple.com/en-us/HT204397

you will find some help.

If this does not cover your situation, months to years ago I had success by changing my ATT email password and re-entering it where applicable.  Good luck.