Was working fine until yesterday and all of a sudden the “Bad Request” started to show up, perhaps Apple changed something on their end this week, due to the recent announcements of software and hardware.
I guess if nothing changed on your side and you are sure nothing was upgraded, and yet it stopped working, then it looks like Apple might be the culprit. Best option: contact them and find out.
Some things you can try on your side:
Anti-virus or firewall application upgrade. Disable that and see if there is any difference.
Restart your router.
eM Client upgrade. Uninstall it and see if using the previous version from the Release History makes any difference.
It seems Apple changed something on the their CardDAV specs, wondering if the emClient team is aware of the changes already.
There you go, Apple off on their own again. Why can’t they just stick with the RFC specifications for CardDAV ?
A temporary solution is to append any imaginary path to the CardDAV url in the account settings: https://stackoverflow.com/questions/57920341/not-able-to-get-current-user-principal-for-user-from-apple-caldav-server?answertab=votes#tab-top