Intermittent idrive backups failing after firmware upgrade to 6.38.5

I'm having weird idrive backup failures from only some PC's behind upgraded Microtik routers.
Downgrading to 6.37.5 and backups will start working again.

I've opened a ticket with the idrive tech, but I have no idea where to start with troubleshooting this issue.
Here's example of one network:
Router firmware update 3/9/2017

PC1
Backup 3/8/2017 backup ok
Backup 3/10/2017 backup failed due to "connection issues"

PC2
Backups still running fine 3/15/2017
Backups started failing 3/16/2017

Downgrade to 6.37.5 firmware and backups start working again.

Don't think a supout.rif is going to help with this.

Any thoughts welcome.