Several .datto and .overlay files in root

There are several large .datto and .overlay files in the root of a linux client box.
I think they are related to old failed backups.
Is it safe to delete them?

These are snapshot backing files created by dattobd, and yes, likely still exist due to not cleaning up after an interrupted backup. You should use dbdctl to destroy the snapshots.
I have no experience with dattobd to assist further.

Those files can just be deleted.

If you want to help, try creating steps to reproduce the problem (leftover snapshot files) with the newest version and post them.