The 3.7 site is of what flavor? Linux?
If linux, there is a backup.php script in code/admin/cli/ that will use backup defaults for system and save to a destination location.
Issued with just: php backup.php
will bring up the help screen for options.
The CLI will take any web application firewall out of the loop when making the backup ... it's just php talking to your DB then.
Once complete, cd to the destination directory and issue:
file -b nameofbackup.mbz
Transfer that file directly to your destination server ... no download or upload. In destination server you could setup a file system repo called 'transfer' that will have a directory in moodledata/repository/transfer/
You could do that via scp.
That method involves no download/upload and avoids any Web Application Firewall ... etc. and positions the .mbz file where it can be restored with no download/upload.
The other thing ... on destination server, turn on debugging all the way to developer. Let's see if there is anything else that moodle might report.
'SoS', Ken