SFTP server periodically stalls

[Terra Frost]

I'm trying to SFTP upload files from one computer to another on a periodic
basis. Most of the time this works great but sometimes (on the hour) the
connection is unsuccessful.

In reviewing the logs...  the SFTP client is able to login via SSH. The
client sends a SSH_MSG_CHANNEL_REQUEST packet for the sftp subsystem and
the server responds with a SSH_MSG_CHANNEL_SUCCESS. The client then sends
an SSH_MSG_CHANNEL_DATA packet containing a SSH_FXP_INIT packet and then...
 nothing. The server *should* be responding with a SSH_FXP_VERSION packet
(wrapped in a SSH_MSG_CHANNEL_DATA packet) but it's not.

On the server end of things the last entry I see in the log files for these
sessions is this:

May 20 09:00:12 sftpacct sshd[22358]: debug3: send packet: type 99


That corresponds to the SSH_MSG_CHANNEL_SUCCESS packet.

Overall it kinda feels to me like that the sftp subsystem just isn't setup
correctly. From my sshd_config file:

Subsystem sftp internal-sftp -f LOCAL7 -l INFO -u 0007


Could that be the source of some of the problems? Where else should I be
looking?

The server is OpenSSH_7.6p1 Ubuntu-4ubuntu0.6.

The full log files are available at https://pastebin.com/ag9q6pVS and
https://pastebin.com/sKYDrJAK.  https://pastebin.com/ag9q6pVS  has "May 20
09:00:12 sftpuser sshd[22287]: User child is on pid 22358" close to the end
and the activity for the new pid is in https://pastebin.com/sKYDrJAK.

Thanks!