Loading the list of folders from the server


I got an error in my desktop client that are showing below message and can’t completed to sync the files.

Error while loading the list of folders from the server.
Error transferring www.xxx.com/remote.php/dave/files/username/ - server replied: Service Unavailable

any one got the same issue? and how to fixed it?

my nextcloud server version is 13.0.2
client is 2.3.3 (build 84) mac version.

I tried logout and re-login in the client and tried restart sync.
and i can’t get the folder list the select what i want sync.