Hi welcome to the forum.
I have had the same thing happen to me on Windows 7 with client version 3.13.0
My brief notes
Started with Blank External drive.
Made client connection to E:
During inital download one file has error connection closed.
https://help.nextcloud.com/t/connection-closed-error/195067
I waited some time and the client had a retry.
I then created a new file on External drive.
Client stuck syncing no network traffic.
Pause client
Error from windows nextcloud has stopped working.
Apon restarting client it started in paused state.
Resumed sync and waited about 30 mins before sync started by its self.