For quite some time now we get support tickets where files with Spanish, Portuguese, French, German, Dutch and other characters in names cannot be backed up. The error is "No such file or directory" which makes no sense.

Presumably the reason is that the backup app requires en_US.UTF-8 locale installed, and in some cases it is missing for some reason.
We are working on a solution these days, so far here is a workaround - install the locale manually. To do so execute these commands in terminal:
sudo locale-gen en_US
sudo locale-gen en_US.UTF-8
sudo update-locale

We received several reports that it helped and backup is going fine.
Let us know if this doesn't help!