I am using 2.9.0 (build 5150) ownCloud-client on Debian 11 from that repos.
To be sure I removed the folders (while ownCloud client is NOT running)
Then I started the client and configured a connection as usual.
I can select the sub-folders to sync without problems. So a connection to the server works.
The in the main ownCloud window I can see the configured synchronisation. But on top of that there is a message that no connection is configured.
In English it looks like this