A backup plan that worked perfectly fine when targeting a local Mac file system/external hard drive failed when targeting a minio server even though it was also running on top of a Mac file system.
It turns out that some files that one of my applications automatically generates (and are therefore not renameable) contain backslashes. So I get a set of errors in my log like:
2019-08-10 18:33:50,905934 [WARN ]: [ CBC ] [ 14 ] Cloud operation error (Operation): Object name contains unsupported characters.
2019-08-10 18:33:50,905974 [WARN ]: [ CBC ] [ 6 ] Cloud operation error (Operation): Object name contains unsupported characters.
2019-08-10 18:33:50,986731 [ERROR]: [ CBB ] [ 5 ] Backup error: /Users/username/Documents/appname/libraries\foo\1.0.1234/foo-1.0.1234.jar
message: Object name contains unsupported characters.
code: 2
2019-08-10 18:33:50,986780 [ERROR]: [ CBB ] [ 5 ] Backup finished: /Users/username/Documents/appname/libraries\foo\1.0.1234/foo-1.0.1234.jar
with errror: Object name contains unsupported characters.And in the GUI there is just a fail with error message “Error on process some files” (which isn’t grammatically correct, and even if it was isn’t very helpful 
)
Minio doesn’t seem likely to resolve this problem, based on reading an older issue in their github https://github.com/minio/minio/issues/5161
So my suggestion here is when connecting to Minio that appropriate character replacements are done (in a recoverable way) so that the backups actually function.
(The same request would hold true for any server that has an incompatible set of allowed characters, I just happened to hit it when trying to use minio on my local network)
A better suggestion, which would solve this issue as well as the issue of potential data leakage, would be to have a metadata file that contains a mapping of original file names to random GUID names that the backups actually get made under.