SFTP intermitent errors

I am using SFTP with Hetzner (yes, I know S3 would probably work better, but that would be another post).

Every know and then I read in server logs:

d’oct. 01 14:57:33 server kopia[2091958]: ERROR connection closed: unrecognized error when opening SFTP file /backup/kopia/kopia.repository.f: connection lost, will retry
d’oct. 01 14:57:33 server kopia[2091958]: ERROR error closing active connection: error closing SFTP client: EOF

I know this host limits connections to 10, so I suspect this is the problem. Is there a way to limit connections?

Reading kopia --help-long there is a

    --num-storage-connections=NUM-STORAGE-CONNECTIONS  
      Number of storage connections

for repository validate-provider, but I don’t think it applies to repository connection.