Syncing CalDav/Nextcloud not working on local lan

REPRODUCIBILITY (% or how often): 100%
BUILD ID = OS VERSION (Settings > About product): 4.0.1.48
HARDWARE (XA2, Xperia 10…): Xperia X
UI LANGUAGE: german
REGRESSION: (compared to previous public release: Yes, No, ?): yes

DESCRIPTION:

When you try to set up or to sync a caldav oder nextcloud account in a local network without Internet connection (intranet) syncing not working and no setup of new account possible. For sure there is a working nextclod server in the intranet. Connecting from PC works, fileupload from mobilephone works.

PRECONDITIONS:

Connect to a local network without internet connection an try to setup a new nextcloud account or try sync a established one.

STEPS TO REPRODUCE:

EXPECTED RESULT:

Possible to sync or to create a new account, even when there is just a local network without internet connection

ACTUAL RESULT:

No Syncing, not possible to set up a new account

ADDITIONAL INFORMATION:

(Please ALWAYS attach relevant data such as logs, screenshots, etc…)