@kugiigi

It's a Let's Encrypt certificate.
And this shows everything is secure.

Yes, this happens, when I add my server to a system account.

I tested yesterday on my tablet too - so this happens my both devices.
My tablet was connected to this server and it worked.
But now it shows the same error.

Devices: Volla Phone, BQ M10 FHD.

Hello,
sorry, my first post here.
Thanks for moving the thread.

Seems that there is a problem with login to Nextcloud.

If I use https:// at server data, than no login is possible (test says "offline" and no news sync works).

If I use the server without https:// login is possible (test calls "online"), but news sync isn't working too.

Some weeks ago, that worked at my configuration. But I had to reinstall the OS, and now there no News on my phone.