@pescepalla Thank’s a lot! Your solution workes on my Xiaomi 5s plus with nextcloud on an own server in my LAN and using DAVx⁵ for synchronisation after I’ve searched two days where the problem comes from. By accident I installed nextcloud 15 and deleted and reconfigured DAVx⁵’s account. So I was not clear if it’s a nextcloud or a DAVx⁵ problem. Finally I found out in the nextcloud logs an entrywebdav Sabre\DAV\Exception\NotAuthenticated: No public access to this resource., No 'Authorization: Basic' header found. Either the client didn't send one, or the server is misconfigured, No 'Authorization: Bearer' header found. Either the client didn't send one, or the server is mis-configured, No 'Authorization: Basic' header found. Either the client didn't send one, or the server is misconfigured
Still misleading, I checked everything related with passwords and accounts.
Android doesn’t sync if it can’t connect to a working captive_portal_https_url, even if you choosed in the internet access settings for your network not to ask the question anymore and the internet works as far, as possible in China. And that with LineageOs. As many people not only using their own local nextcloud installation inside a LAN that is kind of a strange setting with Android in China at least.
Thanks again, @pescepalla !