During restore restart, an I/O error occurred on checkpoint file
Anyone have any ideas? Safe to ignore? Potential time bomb? I'm
  stumped.


It's not a "bomb", this means only that if you want to use RESTORE with RESTART option (to complete interrupted RESTORE ) you'll not be able to do it, i.e. checkpoint file cannot be used and the RESTORE will not be able to skip some passes if they woudl be done prior to interruption.

You can read more about checkpoint file here: Restores using Invalid Backup Default Locations

The cause of this error is that your default backup location becomes temporarily inaccessible.

You can just ignore this error or you can change the default backup location with any local path.