Nextcloud Cal and Carddav using Sailfisch OS an Thunderbird


i am using nextcloud since the fork from owncloud. the carddav and caldav functions doesnt work or work some weeks ( caldav @ sailfisch OS e.g. )

now i got another mysterious malfunction in the lightning calendar app : if i use the direct ip link/url given by the share function in the nextcloud calendar the lightning integration works great. But the Web URL does work an there were no error messages in thunderbird.

works : https://192.168.X.XXX/mycloud/remote.php/dav/calendars/speefak/calender_1/

nextcloud is reachable from the internet and in the webinterface everthing works fine. Also the caldav links ( web URL ) works in my sailfish OS mobile. Any idea why this works via weblinks for my mobile ( so webdav link and server configuration seems correct ) and not for Thunderbird ?