mysql grant option

Your sys-maint@localhost user doesn't have ALL privileges. It's lacking the CREATE TABLESPACE privilege.

If your user really had ALL privileges then when you did a SHOW GRANTS; you'd see:

GRANT ALL PRIVILEGES ON *.* TO 'debian-sys-maint'@'localhost' IDENTIFIED BY PASSWORD '*DA604C65D4EFF216D1F61FCD42726FA881FB6562' WITH GRANT OPTION

To be able to do a GRANT ALL on another or new user, the user issuing the GRANT ALL must themselves have every permission or the grant will fail.

To grant your 'sys-maint@localhost' user CREATE TABLESPACE permissions issue the following command from an account that does have enough rights to do so (e.g. root):

GRANT CREATE TABLESPACE ON *.* TO 'sys-maint@localhost';

Or when logged in as sys-maint@localhost (and this works on MySQL 5.5) do the following:

USE mysql;
UPDATE user SET `Create_tablespace_priv`='Y' 
            WHERE user='sys-maint' AND host='localhost';
FLUSH PRIVILEGES;

Logout then log back in again (QUIT if using the command line mysql client or disconnect if using a GUI client such as HeidiSQL) and sys-maint@localhost should be able to issue the GRANT ALL command.

This works because your sys-maint@localhost already has sufficient privileges to update the user table.

And as always, when tinkering with MySQL permissions, remember to do a FLUSH PRIVILEGES;