@tescher This is a Nextcloud bug, please ask there. See also
SNI stoped working after update
My server delivers the right cert when checking with chrome or the ssl-check-service you recommend. It also worked in previous versions of davdroid. With the new http lib it uses the default host cert and fails.
Duplicate of #205
I am runnung 4.4.2 with the moat recent davdroid version from googles appstore.
There is already a bugfix release 0.5.10.1 that fixes the sni issue. Wait some hours and you will receive the update automatically.