@steve_b
okay, I'm seeing inconsistent behavior here and I haven't been able to pin down why.
router 1 (the originally not working one):
While looking for the debug logs, I noticed that the log entries for success had disappeared. I traced this to a check for boot completion that was failing, preventing the backup from starting (this is a different issue. I also don't know why the boot is no longer finishing).
I manually deleted the booting file so that the backup would run, and it started working. I then walked back the firewall and DNS config (and the debugging stuff that I had added in acb.php) that I had done before to try to get it to fail again, and it would not fail. So, I don't know what changed to make it work.
router 2 (the originally working one):
This one started exhibiting the behavior seen before on router 1 (backups report success in web ui, but they are not occurring). This one does have a backupdebug.txt indicating a timeout on the save. I can still ping and curl acb.netgate.com.