[Snapshot Pro] Managed backup failing

Managed backups was working fine on a multisite install and then stopped working a few weeks ago. I now get the message "SERVICE UNAVAILABLE The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.".

This message comes after about around 15-20% completion.

I changed EXECUTION LIMIT to 1500 seconds.
I changed MEMORY LIMIT to 99999.

I'm seeing these errors in the apache error log:
[Sun Jun 17 11:06:52.429038 2018] [proxy_fcgi:error] [pid 2724] [client 127.0.0.1:58578] AH01067: Failed to read FastCGI header, referer: https://zzzzzzz.com/wp-admin/network/admin.php?page=snapshot_pro_managed_backups&snapshot-action=backup
[Sun Jun 17 11:06:52.430105 2018] [proxy_fcgi:error] [pid 2724] (104)Connection reset by peer: [client 127.0.0.1:58578] AH01075: Error dispatching request to : , referer: https://zzzzzzz.com/wp-admin/network/admin.php?page=snapshot_pro_managed_backups&snapshot-action=backup

WPMUDEV support access has been enabled in case you want to have a closer look.

Any ideas on how to troubleshoot?