No connection to Nextcloud at https://...... but can connect through browser

Hi, the Nextcloud desktop app gives the following error and won’t sync, whereas I can successfuly connect to the server and see my files through Chrome browser:
No connection to Nextcloud at https://…
I couldn’t figure it out. Are there some logs where I can check the details of the error?
Edit: while setting up sync on the desktop app, the total size of my files can be seen correctly, so I guess the app has some access to the server. But no sync happens after setup.
Edit 2: I have tried removing and re-adding my account, and removing and re-installing the Nextcloud app. It didn’t help.

Operating system: Xubuntu 20.04 x64

Details of the Nextcloud installation:
(Installed using apt-get)
Nextcloud version 2.6.2-1build1
Using Qt 5.12.8, built against Qt 5.12.5
Using ‘OpenSSL 1.1.1f 31 Mar 2020’

Thanks in advance,