That is weird, I had no issues with DAVx5 after updating. The URL is still the same and works, without the /so/
Maybe an issue with your reverse proxy?
What happens with other CalDAV clients like Thunderbird?
With me its actually:
https://mail.mydomain.ch/SOGo/dav/<ACCOUNT>/...
That “dav” seems missing with you.
One important thing to remember: If your account is using 2FA, you must use an app password now, you cannot use your login password anymore!
(see the grey box on top of the forum overview)