The user "ftpuser" is a user account used by FRITZ!Boxes in older FRITZ!OS versions, and it could be used to access storage connected to the FRITZ!Box.

By default the "ftpuser" was authorized to "Access to NAS contents" and had read and write permission for all storage available on the FRITZ!Box. For security reasons, the "ftpuser" could not access the FRITZ!Box over the internet. By default, the password for "ftpuser" was the same as the password for the FRITZ!Box user interface.

If there is still a user account "ftpuser" in your FRITZ!Box, you can delete this account under "System > FRITZ!Box Users".