Server not available with account in Nextcloud app

I’m getting Server not available error in Nextcloud app (also in its Dev variant) for server and account. Some vital details for the account are communicated though, but not the files.

I can see account details (username, quotas and usage), but not the content of Files.

Nextcloud app 3.26.0
on SM-G975F

Any suggestion what to check for?

One of my four accounts is experiencing this issue. The error message appears with the same account on my LG G5 device, too. Despite the error message, sync is working there and I could upload and download files to and from the cloud.