I’ve the same issue. With Android 8.0.0 and Joplin app 1.0.141, connecting to Nextcloud 13 on Apache 2.2. The server is running on my LAN with a local certificate chain. The root certificate is installed in the Android certificate store.
If I use the web browser, I can connect no problem.
In Joplin, testing the sync settings, results in a “Network request failed” error. Whilst it’s attempting the connection, I can see the connection attempts on the server, with multiple socket connections in TIME-WAIT.
Unfortunately, the log doesn’t give much away…
10-27T11:33:43,10,""“TypeError: Network request failed
I’ve gone through other tickets and seem to have everything else covered, so have run out of ideas.